Add BasicAuthorizationInterceptor

This commit adds a `ClientHttpRequestInterceptor` that applies a BASIC
authorization header for each request.

It can be used as follows:

```
BasicAuthorizationInterceptor basicAuthorization =
    new BasicAuthorizationInterceptor("user", "secret");
restTemplate.getInterceptors().add(basicAuthorization);
```

Issue: SPR-14412

spring-web/src/main/java/org/springframework/http/client/BasicAuthorizationInterceptor.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2002-2016 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.client;
+
+import java.io.IOException;
+import java.nio.charset.Charset;
+
+import org.springframework.http.HttpRequest;
+import org.springframework.util.Assert;
+import org.springframework.util.Base64Utils;
+
+/**
+ * {@link ClientHttpRequestInterceptor} to apply a BASIC authorization header.
+ *
+ * @author Phillip Webb
+ * @since 4.3.1
+ */
+public class BasicAuthorizationInterceptor implements ClientHttpRequestInterceptor {
+
+	private static final Charset UTF_8 = Charset.forName("UTF-8");
+
+	private final String username;
+
+	private final String password;
+
+	public BasicAuthorizationInterceptor(String username, String password) {
+		Assert.hasLength(username, "Username must not be empty");
+		this.username = username;
+		this.password = (password == null ? "" : password);
+	}
+
+	@Override
+	public ClientHttpResponse intercept(HttpRequest request, byte[] body,
+			ClientHttpRequestExecution execution) throws IOException {
+		String token = Base64Utils
+				.encodeToString((this.username + ":" + this.password).getBytes(UTF_8));
+		request.getHeaders().add("Authorization", "Basic " + token);
+		return execution.execute(request, body);
+	}
+
+}