Return stream from fetch
Fixes to work from latest master merge.

- Ignore EventEmitter and Stream prototype methods and properties when copying
  a message to fetches[i]._msg
- Do not re-instantiate a message that is already cached on a fetches[i]._msg
- Handle expired or invalid access token more properly, without parsing the
  line that may represent the JSON status 400 response from gmail.
- Encapsulate streamification