Merge branch '5.1' into 5.x

* 5.1:
  Update versions in composer.json
  [Mime] Fix serialization of RawMessage

Author: nicolas-grekas

composer.json

@@ -37,5 +37,5 @@
         ]
     },
     "minimum-stability": "dev",
-    "version": "5.2-dev"
+    "version": "5.2.x-dev"
 }