[MRG+1] Add ability to use FormRequest in contracts

[StasDeep]

With current implementation of contracts, it is basically not possible to create a generic @form contract, but I think we should at least allow to add formdata in custom contracts.

Closes #3382.

[codecov]

Codecov Report

Merging #3383 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3383      +/-   ##
==========================================
+ Coverage   84.46%   84.48%   +0.01%     
==========================================
  Files         167      167              
  Lines        9362     9369       +7     
  Branches     1390     1392       +2     
==========================================
+ Hits         7908     7915       +7     
  Misses       1199     1199              
  Partials      255      255

Impacted Files	Coverage Δ
scrapy/contracts/__init__.py	80.5% <100%> (+0.86%)	⬆️
scrapy/spidermiddlewares/offsite.py	100% <0%> (ø)	⬆️

[kmike]

Hey @StasDeep! What do you think about adding request_cls attribute to Contract instead, so that one can define a contract which uses FormRequest, SplashRequest or any other Request class? I don't quite like hardcoding formdata argument.

[StasDeep]

Hi @kmike! Thanks for the idea, that sounds much better indeed. I've implemented request_cls feature. Could you please review it?

[kmike]

I was thinking about doing it in a different way:

class CustomFormContract(Contract):
    name = 'custom_form'
    request_cls = FormRequest

    def adjust_request_args(self, args):
         args['formdata'] = {'name': 'scrapy'}
         return args

Not passing request_cls in args looks cleaner to me, because request_cls is not really a keyword argument passed to a request initializer.

That said, if it is not an request arg, we'd need some rule to determine which class to use if there are several contracts in a chain. Something like "take last explicitly defined request_cls".

[StasDeep]

@kmike could you please review the PR?

[kmike]

Looks good, thanks @StasDeep!