proto: change contentType to bytes format

This works around the non-UTF8 character error that is emitted when parsing
some mercury responses by changing the type of the contentType field to bytes.

Derrived from Hexxeh/spotify-websocket-api@e6b2d8772d

Fixes TooTallNate/node-spotify-web#11
Merge pull request #14 from adammw/proto-suppress-error

proto: change contentType to bytes format