Escape header values before sending to GRPC

HTTP header values can only contain ISO-8859-1 characters. Google-Gax should "percent-encode" header values that contain characters outside of this character range.

This would fix https://github.com/firebase/firebase-functions/issues/463 by correctly encoding [`parent`](https://github.com/googleapis/nodejs-firestore/blob/master/dev/src/v1/firestore_client.js#L1075).