Support of HTTP persistent connections for JDK client

Prior to this commit, HTTP clients relying on the JDK HTTP client would
not properly reuse existing TCP connections (i.e. HTTP 1.1 persisten
connection). The SimpleClientHttpResponse would close the actual connection once the
response is handled.

As explained in the JDK documentation
(http://docs.oracle.com/javase/8/docs/technotes/guides/net/http-keepalive.html)
HTTP clients should do the following to allow resource reuse:

* consume the whole HTTP response content
* close the response inputstream once done

This commit makes sure that the response content is
totally drained and then the stream closed (and not the connection).

Issue: SPR-14040

spring-core/src/main/java/org/springframework/util/StreamUtils.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * 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.
@@ -131,6 +131,25 @@ public abstract class StreamUtils {
 		return byteCount;
 	}
 
+	/**
+	 * Drain the remaining content of the given InputStream.
+	 * Leaves the InputStream open when done.
+	 * @param in the InputStream to drain
+	 * @return the number of bytes read
+	 * @throws IOException in case of I/O errors
+	 * @since 4.3.0
+	 */
+	public static int drain(InputStream in) throws IOException {
+		Assert.notNull(in, "No InputStream specified");
+		byte[] buffer = new byte[BUFFER_SIZE];
+		int bytesRead = -1;
+		int byteCount = 0;
+		while ((bytesRead = in.read(buffer)) != -1) {
+			byteCount += bytesRead;
+		}
+		return byteCount;
+	}
+
 	/**
 	 * Return an efficient empty {@link InputStream}.
 	 * @return a {@link ByteArrayInputStream} based on an empty byte array