fetch() isn't handling extra_parameters if it's a string

[vclayton]

So the docs say extra_parameters is an array, but it turns out it'll allow a string. In that case the string should become the request body (for a POST/PUT). Not exactly sure what it should do for a GET.

[vclayton]

Still don't know what should happen with a GET, but POST/PUT both work properly now. No support for file uploads though.