Merge branch '4.4' into 5.1

* 4.4:
  Update versions in composer.json
  [Mime] Fix serialization of RawMessage

Author: nicolas-grekas

composer.json

@@ -33,5 +33,5 @@
         ]
     },
     "minimum-stability": "dev",
-    "version": "5.1-dev"
+    "version": "5.1.x-dev"
 }