Fixes #202: $.fetch support for multipart

[friday]

This implements the suggestion in #202 in order to support multipart in $.fetch.

There hasn't been any discussion in #202, so this may not be desirable. After all it's a "breaking" change.

[LeaVerou]

This seems to be adding an unrelated check to fix a different problem than what that check is testing. It's a heuristic, and not sure if it's a good heuristic. And as you mention, it could break some legit use cases.

You can already pass specific headers, via the headers parameter. So, in theory, headers: { "Content-Type": undefined } should fix this problem. If not, I'd be glad to merge a PR along these lines.

Thanks for contributing, and I hope this doesn’t put you off future contributions!

[friday]

No worries :) That's your job as a maintainer, and it was probably a good call. I'll try to justify this PR later.