Fixing issue #100 withHeader does not remove old header value

Author: pdscopes

src/MessageTrait.php

@@ -204,6 +204,9 @@ public function withHeader($header, $value)
         $normalized = strtolower($header);
 
         $new = clone $this;
+        if($new->hasHeader($header)) {
+            unset($new->headers[$new->headerNames[$normalized]]);
+        }
         $new->headerNames[$normalized] = $header;
         $new->headers[$header]         = $value;