Naming, immutable array manipulation

This is a sqashed version to add back the changes from reactphp#37:
- 2ab98d2
- 75ff84a

src/RequestHeaderParser.php

@@ -44,12 +44,13 @@ public function parseRequest($data)
             parse_str($queryString, $parsedQuery);
         }
 
-        $headers = $psrRequest->getHeaders();
-        array_walk($headers, function(&$val) {
+        $headers = array_map(function($val) {
             if (1 === count($val)) {
                 $val = $val[0];
             }
-        });
+
+            return $val;
+        }, $psrRequest->getHeaders());
 
         $request = new Request(
             $psrRequest->getMethod(),