Add HttpRequestProvider

[Gallaecio]

To mirror response counterparts, I only implemented a provider for HttpRequest, and not for HttpRequestBody or HttpRequestHeaders. Is that OK, or should we provide more granular providers? It would be trivial to do, but if we do I wonder if I should do the same with response page input classes as well.

To-do:

• Get Add from_bytes_dict to HttpRequestHeaders web-poet#191 merged, release a new version of web-poet, and require that new version in setup.py.

[codecov]

Codecov Report

Merging #173 (db2d0b4) into master (aba2b74) will decrease coverage by 2.85%.
The diff coverage is 100.00%.

❗ Current head db2d0b4 differs from pull request most recent head 644e5be. Consider uploading reports for the commit 644e5be to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
- Coverage   85.85%   83.00%   -2.85%     
==========================================
  Files          14       14              
  Lines         813      818       +5     
==========================================
- Hits          698      679      -19     
- Misses        115      139      +24     

Files	Coverage Δ
scrapy_poet/page_input_providers.py	93.33% <100.00%> (-6.67%)	⬇️

... and 3 files with indirect coverage changes