Add support for multipart/form-data to FormRequest

[alexandresgf]

Fixes #1897, closes #1954

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.01%. Comparing base (1d11ea3) to head (6b6af57).
Report is 8 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #6332       +/-   ##
===========================================
- Coverage   83.92%   43.01%   -40.92%     
===========================================
  Files         161      161               
  Lines       11972    12010       +38     
  Branches     1865     1931       +66     
===========================================
- Hits        10048     5166     -4882     
- Misses       1588     6421     +4833     
- Partials      336      423       +87     

Files	Coverage Δ
scrapy/http/request/form.py	19.40% <ø> (-78.36%)	⬇️

... and 141 files with indirect coverage changes

[wRAR]

Doesn't look like this changes anything except Content-Type, is this correct?

[alexandresgf]

Doesn't look like this changes anything except Content-Type, is this correct?

Right, as suggested by the OP in the ticket, it should be a flag, in this case is_multipart, which changes the Content-Type to multipart/form-data. I also added a new parameter to FormRequest constructor.

[wRAR]

The format of multipart/form-data requests is very different from the application/x-www-form-urlencoded one, see e.g. https://www.w3.org/TR/html401/interact/forms.html#h-17.13.4 or the previous PR on this.