Support bytes fields in path parameter

[yugui]

Should decode percent encoding to support bytes fields

[IceMan81]

@yugui Are oneofs supported?

[yugui]

@IceMan81 good point. Since the API of oneof in go protobuf has changed, we need to care of that too.

[IceMan81]

@yugui Explains why it didnt work when I tried a oneof today; code generation ran without any errors, but the gateway code was unable to map the request params to the protobuf fields.

[tbg]

This issue is referenced from the README as covering enum support, which isn't mentioned here. It should be added or filed as a new issue so that it doesn't get lost.