Reintroduce typecasting to the Protobuf serializer

The Protobuf serializer used to handle coercing values to Protobuf field
types. It was (incorrectly) removed, which broke things that relied on
passing values directly to non-persistance methods (i.e. creating
requests with values other than the internal attributes) or values
directly from web requests (i.e. the Rails params hash).

Reintroduce typecasting using the Active Attr typecasters. Add specs to
ensure things work as expected.
Merge pull request #19 from liveh2o/typecasting

Reintroduce typecasting to the Protobuf serializer