Improve HTTP caching flexiblity

This commit improves HTTP caching defaults and flexibility in
Spring MVC.

1) Better default caching headers

The `WebContentGenerator` abstract class has been updated with
better HTTP defaults for HTTP caching, in line with current
browsers and proxies implementation (wide support of HTTP1.1, etc);
depending on the `setCacheSeconds` value:

* sends "Cache-Control: max-age=xxx" for caching responses and
do not send a "must-revalidate" value by default.
* sends "Cache-Control: no-store" or "Cache-Control: no-cache"
in order to prevent caching

Other methods used to set specific header such as
`setUseExpiresHeader` or `setAlwaysMustRevalidate` are now deprecated
in favor of `setCacheControl` for better flexibility.
Using one of the deprecated methods re-enables previous HTTP caching
behavior.

This change is applied in many Handlers, since
`WebContentGenerator` is extended by `AbstractController`,
`WebContentInterceptor`, `ResourceHttpRequestHandler` and others.

2) New CacheControl builder class

This new class brings more flexibility and allows developers
to set custom HTTP caching headers.

Several strategies are provided:

* `CacheControl.maxAge(int)` for caching responses with a
"Cache-Control: max-age=xxx" header
* `CacheControl.noStore()` prevents responses from being cached
with a "Cache-Control: no-store" header
* `CacheControl.noCache()` forces caches to revalidate the cached
response before reusing it, with a "Cache-Control: no-store" header.

From that point, it is possible to chain method calls to craft a
custom CacheControl instance:

```
CacheControl cc = CacheControl.maxAge(1, TimeUnit.HOURS)
    .cachePublic().noTransform();
```

3) Configuring HTTP caching in Resource Handlers

On top of the existing ways of configuring caching mechanisms,
it is now possible to use a custom `CacheControl` to serve
resources:

```
@Configuration
public class MyWebConfig extends WebMvcConfigurerAdapter {

  @Override
  public void addResourceHandlers(ResourceHandlerRegistry registry) {
    CacheControl cc = CacheControl.maxAge(1, TimeUnit.HOURS);
    registry.addResourceHandler("/resources/**)
            .addResourceLocations("classpath:/resources/")
            .setCacheControl(cc);
  }
}
```

or

```
<mvc:resources mapping="/resources/**" location="classpath:/resources/">
  <mvc:cachecontrol max-age="3600" cache-public="true"/>
</mvc:resources>
```

Issue: SPR-2779, SPR-6834, SPR-7129, SPR-9543, SPR-10464

spring-web/src/test/java/org/springframework/http/CacheControlTests.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2002-2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.http;
+
+import org.hamcrest.Matchers;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+import java.util.concurrent.TimeUnit;
+
+/**
+ * @author Brian Clozel
+ */
+public class CacheControlTests {
+
+	private static final String CACHE_CONTROL_HEADER = "Cache-Control";
+
+	@Test
+	public void emptyCacheControl() throws Exception {
+		CacheControl cc = CacheControl.empty();
+		assertThat(cc.getHeaderValue(), Matchers.nullValue());
+	}
+
+	@Test
+	public void maxAge() throws Exception {
+		CacheControl cc = CacheControl.maxAge(1, TimeUnit.HOURS);
+		assertThat(cc.getHeaderValue(), Matchers.equalTo("max-age=3600"));
+	}
+
+	@Test
+	public void maxAgeAndDirectives() throws Exception {
+		CacheControl cc = CacheControl.maxAge(3600, TimeUnit.SECONDS).cachePublic().noTransform();
+		assertThat(cc.getHeaderValue(), Matchers.equalTo("max-age=3600, no-transform, public"));
+	}
+
+	@Test
+	public void maxAgeAndSMaxAge() throws Exception {
+		CacheControl cc = CacheControl.maxAge(1, TimeUnit.HOURS).sMaxAge(30, TimeUnit.MINUTES);
+		assertThat(cc.getHeaderValue(), Matchers.equalTo("max-age=3600, s-maxage=1800"));
+	}
+
+	@Test
+	public void noCachePrivate() throws Exception {
+		CacheControl cc = CacheControl.noCache().cachePrivate();
+		assertThat(cc.getHeaderValue(), Matchers.equalTo("no-cache, private"));
+	}
+
+	@Test
+	public void noStore() throws Exception {
+		CacheControl cc = CacheControl.noStore();
+		assertThat(cc.getHeaderValue(), Matchers.equalTo("no-store"));
+	}
+}