Use HTTPS for external links wherever possible

See gh-16316

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/server/HttpTunnelServer.java

@@ -39,7 +39,7 @@ import org.springframework.util.Assert;
 
 /**
  * A server that can be used to tunnel TCP traffic over HTTP. Similar in design to the
- * <a href="http://xmpp.org/extensions/xep-0124.html">Bidirectional-streams Over
+ * <a href="https://xmpp.org/extensions/xep-0124.html">Bidirectional-streams Over
  * Synchronous HTTP (BOSH)</a> XMPP extension protocol, the server uses long polling with
  * HTTP requests held open until a response is available. A typical traffic pattern would
  * be as follows:

spring-boot-devtools/src/test/java/org/springframework/boot/devtools/classpath/ClassPathFileSystemWatcherTests.java

@@ -72,7 +72,7 @@ public class ClassPathFileSystemWatcherTests {
 		Map<String, Object> properties = new HashMap<String, Object>();
 		File folder = this.temp.newFolder();
 		List<URL> urls = new ArrayList<URL>();
-		urls.add(new URL("http://spring.io"));
+		urls.add(new URL("https://spring.io"));
 		urls.add(folder.toURI().toURL());
 		properties.put("urls", urls);
 		MapPropertySource propertySource = new MapPropertySource("test", properties);

spring-boot-devtools/src/test/java/org/springframework/boot/devtools/restart/ChangeableUrlsTests.java

@@ -61,7 +61,7 @@ public class ChangeableUrlsTests {
 
 	@Test
 	public void httpUrl() throws Exception {
-		URL url = new URL("http://spring.io");
+		URL url = new URL("https://spring.io");
 		assertThat(ChangeableUrls.fromUrls(url).size(), equalTo(0));
 	}