default mimetype causes 400 errors with Google Calendar Batch API

[hexsprite]

https://github.com/wapisasa/batchelor/blob/master/lib/batchelor.js#L49

the type is listed as multipart/mixed; -- NOTE: trailing semi-colon

I get 400 error from Google Calendar Batch API, but if I override the value without the semi-colon it works fine.

[jamesmhaley]

If you call other endpoints without the ; does it fail??

Feel free to create a PR with a solution and myself or @bradvogel will merge it.