Use CONNECT method to contact Crawlera proxy

[redapple]

Scrapy can connect fine with Crawlera using CONNECT method. I'm not 100% sure we need to have this on by default though. This could be switchable with a new CRAWLERA_USE_HTTP_CONNECT setting of some sort.

[codecov-io]

Codecov Report

Merging #44 into master will decrease coverage by 0.09%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #44     +/-   ##
=========================================
- Coverage   93.75%   93.65%   -0.1%     
=========================================
  Files           2        2             
  Lines         128      126      -2     
=========================================
- Hits          120      118      -2     
  Misses          8        8

Impacted Files	Coverage Δ
scrapy_crawlera/middleware.py	93.54% <ø> (-0.11%)	:arrow_down:

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[asadurski]

I think we should make the use of CONNECT configurable, perhaps through a key in _settings (https://github.com/scrapy-plugins/scrapy-crawlera/blob/master/scrapy_crawlera/middleware.py#L24)?

[ghost]

@eLRuLL - Sorry for accidentally requesting review there. Github recommended you as a reviewer, but I misread the UI to mean that you had recommendations. :)

[eLRuLL]

no problem @cathalgarvey I accept it btw 😄

[dangra]

@cathalgarvey master branch builds are broken since this PR was merged. Can you take a look please?

[ghost]

Sure @dangra