Ensure we only store the _values_ of arrays provided as headers

Doing so ensures that when we merge values via `withAddedHeader()`, we
retain the original values when key collissions might normally occur.

Fixes compatibility with PSR-7.

Author: weierophinney

src/MessageTrait.php

@@ -396,7 +396,7 @@ private function filterHeaderValue($values)
             HeaderSecurity::assertValid($value);
 
             return (string) $value;
-        }, $values);
+        }, array_values($values));
     }
 
     /**