Accept multiple params in content-type. Fixes #362

[fabiangehring]

No description provided.

[fabiangehring]

Facing the same problem with multi-param content types as in #362.
Content-Type: application/json; odata.metadata=minimal; odata.streaming=true; charset=utf-8

see here:
https://stackoverflow.com/questions/37184862/r-package-httr-error-in-one-liner-get-after-update-form-0-5-to-1-1-0