Add provider for RequestUrl

[Gallaecio]

We might want to add 1 more test once we merge #62, featuring a page object that uses additional requests, to make sure those work even if the original callback gets a dummy request (they should, but to me it seems worth testing nonetheless).

[codecov]

Codecov Report

Merging #76 (d24a7d9) into master (e4589e6) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head d24a7d9 differs from pull request most recent head 05f1bfd. Consider uploading reports for the commit 05f1bfd to get more accurate results

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
+ Coverage   97.76%   97.79%   +0.02%     
==========================================
  Files          10       10              
  Lines         448      454       +6     
==========================================
+ Hits          438      444       +6     
  Misses         10       10              

Impacted Files	Coverage Δ
scrapy_poet/middleware.py	100.00% <100.00%> (ø)
scrapy_poet/page_input_providers.py	95.38% <100.00%> (+0.38%)	⬆️

[kmike]

👍