Relax use of spring.session.store-type

This commit makes the "spring.session.store-type" property optional,
adding an additional check when it is not present that validates only
one supported implementation is available on the classpath.

As Spring Session has been modularized, the chance that multiple
implementations are available on the classpath are lower. When only
one implementation is present, we attempt to auto-configure it. When
more than one implementation is present and no session store is
configured, a NonUniqueSessionRepositoryException is thrown.

Closes gh-9863

spring-boot-test/src/main/java/org/springframework/boot/test/context/HideClassesClassLoader.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2012-2017 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.boot.test.context;
+
+import java.net.URL;
+import java.net.URLClassLoader;
+
+/**
+ * Test {@link URLClassLoader} that hides configurable classes.
+ *
+ * @author Stephane Nicoll
+ * @since 2.0.0
+ */
+public class HideClassesClassLoader extends URLClassLoader {
+
+	private final Class<?>[] hiddenClasses;
+
+	public HideClassesClassLoader(Class<?>... hiddenClasses) {
+		super(new URL[0], HideClassesClassLoader.class.getClassLoader());
+		this.hiddenClasses = hiddenClasses;
+	}
+
+	@Override
+	protected Class<?> loadClass(String name, boolean resolve)
+			throws ClassNotFoundException {
+		for (Class<?> hiddenClass : this.hiddenClasses) {
+			if (name.equals(hiddenClass.getName())) {
+				throw new ClassNotFoundException();
+			}
+		}
+		return super.loadClass(name, resolve);
+	}
+
+}