Solving the issue #1387

src/Composer/Util/StreamContextFactory.php

@@ -67,12 +67,19 @@ final class StreamContextFactory
                     $auth .= ':' . $proxy['pass'];
                 }
                 $auth = base64_encode($auth);
+                $authHeader = "Proxy-Authorization: Basic {$auth}";
 
                 // Preserve headers if already set in default options
                 if (isset($defaultOptions['http']['header'])) {
-                    $defaultOptions['http']['header'][] = "Proxy-Authorization: Basic {$auth}";
+                     if (is_array($defaultOptions['http']['header']))
+                        $defaultOptions['http']['header'][] = $authHeader;
+                    else
+                        $defaultOptions['http']['header'] = array(
+                            $defaultOptions['http']['header'],
+                            $authHeader
+                        );
                 } else {
-                    $options['http']['header'] = array("Proxy-Authorization: Basic {$auth}");
+                    $options['http']['header'] = array($authHeader);
                 }
             }
         }

tests/Composer/Test/Util/StreamContextFactoryTest.php

@@ -126,6 +126,20 @@ class StreamContextFactoryTest extends \PHPUnit_Framework_TestCase
         }
     }
 
+    public function testHttpProxyWithContentTypeHeader() {
+         $_SERVER['http_proxy'] = 'http://username:password@proxyserver.net';
+
+        $context = StreamContextFactory::getContext(array('http' => array('method' => 'GET', 'header' => 'Content-Type: application/json')));
+        $options = stream_context_get_options($context);
+
+        $this->assertEquals(array('http' => array(
+            'proxy' => 'tcp://proxyserver.net:80',
+            'request_fulluri' => true,
+            'method' => 'GET',
+            'header' => array("Content-Type: application/json", "Proxy-Authorization: Basic " . base64_encode('username:password'))
+        )), $options);
+    }
+
     public function dataSSLProxy()
     {
         return array(