Fix stale request object when an explicit serializer type is passed

[zeppelin]

Fixes #885. (Was likely introduced here)

Although it does solve the issue in the least invasive way, the bug could persist in setups where the serialize() method is overridden, because while the request argument itself is documented, the doc suggests it's fine to throw it away. Also, it doesn't quite fix the underlying issue, namely that serializer & serializer-registry state isn't reset across requests. For example, when serializerRegistry.serializerFor is used to obtain the serializer, the request object must be assigned manually to the serializer, otherwise it'd break the same as before.

Nevertheless, all other solutions would end up being much more complex and have more likelihood of breaking some Mirage setups.

[zeppelin]

Ping @IanVS :)