[MRG+1] data URI download handler.

[ArturGaspar]

GitHub does not let me change head branch in #2175, so I have to open a new PR.

Fixes #2156

[codecov-io]

Codecov Report

Merging #2334 into master will increase coverage by 0.04%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master    #2334      +/-   ##
==========================================
+ Coverage    83.9%   83.95%   +0.04%     
==========================================
  Files         161      162       +1     
  Lines        8893     8935      +42     
  Branches     1312     1317       +5     
==========================================
+ Hits         7462     7501      +39     
  Misses       1176     1176              
- Partials      255      258       +3

Impacted Files	Coverage Δ
scrapy/settings/default_settings.py	98.62% <ø> (ø)	✅
scrapy/core/downloader/handlers/datauri.py	93.33% <93.33%> (ø)
scrapy/core/downloader/handlers/http11.py	90.83% <0%> (-0.31%)	❌
scrapy/utils/misc.py	91.66% <0%> (ø)	✅
scrapy/core/downloader/handlers/ftp.py	98.21% <0%> (+0.03%)	✅
scrapy/utils/python.py	85.62% <0%> (+0.17%)	✅
scrapy/downloadermiddlewares/stats.py	92.3% <0%> (+0.3%)	✅
scrapy/utils/response.py	85.41% <0%> (+0.31%)	✅
... and 1 more

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[redapple]

@ArturGaspar , w3lib 1.17 will be released soon (see scrapy/w3lib#87)

[redapple]

@ArturGaspar , I relaunched the Travis builds with w3lib 1.17. All green :)

[dangra]

FileDownloadHandler class is in a module named file, HTTPDownloadHanlder is in a module named http so I would name the file datauri.py instead of data.py, it is analogous to class name DataUriDownloadHanlder.

[dangra]

This PR should bump required minimum w3lib version to 1.17 in scrapy's setup.py

[kmike]

Could you please rebase it on master?

There was a few other places w3lib version should be bumped, and a few other pull requests which needed this bump, so the required w3lib version is now increased in master.

[redapple]

Looks good to me.
@dangra , @kmike , what do you think?

[kmike]

@ArturGaspar looks good!

Could you please add a test (or modify an existing test) to check other Response attributes: url, headers?

[kmike]

Looks good, thanks @ArturGaspar!