Dont send content-type when body is FormData

[fabricionaweb]

The browser has the ability to treat the content-type when you're omitting them for a formData content, to a file upload for instance (and it's my case)

As discussed in #28 follow my contribution :)

Im actually using the provide PR's solution in a side-project and so far worked like a charm

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[fabricionaweb]

@googlebot I signed it!

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[developit]

Thanks for the great PR @fabricionaweb! I swapped out the instanceof check for duck typing to save some bytes, but everything else is on master and will be released today.