HttpClientConnection: Register connection socket factory for http

It is necessary to register a socket connection factory to prevent the
"http protocol is not supported" error when connecting over a proxy.

Change-Id: Iedf554acef841f52c1f2e3401ef0a0583ac5253b
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.http.apache/src/org/eclipse/jgit/transport/http/apache/HttpClientConnection.java

@@ -89,6 +89,7 @@ import org.apache.http.client.methods.HttpUriRequest;
 import org.apache.http.config.Registry;
 import org.apache.http.config.RegistryBuilder;
 import org.apache.http.conn.socket.ConnectionSocketFactory;
+import org.apache.http.conn.socket.PlainConnectionSocketFactory;
 import org.apache.http.conn.ssl.SSLConnectionSocketFactory;
 import org.apache.http.conn.ssl.X509HostnameVerifier;
 import org.apache.http.impl.client.HttpClientBuilder;
@@ -157,7 +158,9 @@ public class HttpClientConnection implements HttpConnection {
 				clientBuilder.setSSLSocketFactory(sslConnectionFactory);
 				Registry<ConnectionSocketFactory> registry = RegistryBuilder
 						.<ConnectionSocketFactory> create()
-						.register("https", sslConnectionFactory).build();
+						.register("https", sslConnectionFactory)
+						.register("http", PlainConnectionSocketFactory.INSTANCE)
+						.build();
 				clientBuilder.setConnectionManager(
 						new BasicHttpClientConnectionManager(registry));
 			}