Fixed bug with map field stringifying

[andrewosh]

Map fields are currently stringified with an extra map token, which is invalid syntax, i.e.:

message Data {
  map<string, string> something = 1;
}

When parsed + stringified produces:

message Data {
  map<string, string> map something = 1;
}

This PR fixes stringify for map fields, and includes a test that fails on master.