Two changes have been applied:
* prefilters are now applied before data is converted to a string;
this allows prefilters to disable such a conversion
* a prefilter for binary data is added; it disables data conversion
for non-string non-plain-object `data`; for `FormData` bodies, it
removes manually-set `Content-Type` header - this is required
as browsers need to append their own boundary to the header
Ref gh-4150
Closes gh-5197
a7ed9a7b63