Tweak method signatures in HttpComponentsClientHttpRF

This change replaces CloseableHttpClient with HttpClient in the
methods signatures of HttpComponentsClientHttpRequestFactory.
This allows 3rd party libraries such as Spring OAth, which configure
an instance of HttpComponentsClientHttpRequestFactory on the
RestTemplate to remain compatible with both Spring Framework 3/4

Issue: SPR-11053

Author: rstoyanchev

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

@@ -74,27 +74,37 @@ public HttpComponentsClientHttpRequestFactory() {
 	/**
 	 * Create a new instance of the {@code HttpComponentsClientHttpRequestFactory}
 	 * with the given {@link HttpClient} instance.
+	 * <p>
+	 * As of Spring Framework 4 the given client is expected to be of type
+	 * CloseableHttpClient (requiring HttpClient 4.3+).
+	 *
 	 * @param httpClient the HttpClient instance to use for this request factory
 	 */
-	public HttpComponentsClientHttpRequestFactory(CloseableHttpClient httpClient) {
+	public HttpComponentsClientHttpRequestFactory(HttpClient httpClient) {
 		Assert.notNull(httpClient, "'httpClient' must not be null");
-        this.httpClient = httpClient;
+		Assert.isInstanceOf(CloseableHttpClient.class, httpClient, "'httpClient' is not of type CloseableHttpClient");
+        this.httpClient = (CloseableHttpClient) httpClient;
 	}
 
 
     /**
      * Set the {@code HttpClient} used for
+	 * <p>
+	 * As of Spring Framework 4 the given client is expected to be of type
+	 * CloseableHttpClient (requiring HttpClient 4.3+).
+	 *
      * {@linkplain #createRequest(URI, HttpMethod) synchronous execution}.
      */
-    public void setHttpClient(CloseableHttpClient httpClient) {
-        this.httpClient = httpClient;
+    public void setHttpClient(HttpClient httpClient) {
+		Assert.isInstanceOf(CloseableHttpClient.class, httpClient, "'httpClient' is not of type CloseableHttpClient");
+        this.httpClient = (CloseableHttpClient) httpClient;
     }
 
     /**
 	 * Return the {@code HttpClient} used for
 	 * {@linkplain #createRequest(URI, HttpMethod) synchronous execution}.
 	 */
-	public CloseableHttpClient getHttpClient() {
+	public HttpClient getHttpClient() {
 		return this.httpClient;
 	}
 
@@ -130,7 +140,7 @@ public void setBufferRequestBody(boolean bufferRequestBody) {
 
 	@Override
 	public ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException {
-		CloseableHttpClient client = getHttpClient();
+		CloseableHttpClient client = (CloseableHttpClient) getHttpClient();
 		Assert.state(client != null, "Synchronous execution requires an HttpClient to be set");
 		HttpUriRequest httpRequest = createHttpUriRequest(httpMethod, uri);
 		postProcessHttpRequest(httpRequest);