fix TestClient to preserve raw URI

[liborjelinek]

Summary of Changes

When following raw URL recipe based on RAW_URI, it breaks routing with TestClient because TestClient hard-code RAW_URI CGI variable to /.

Related Issues

2157

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[codecov[bot]]

Codecov Report

Merging #2159 (074a182) into master (7191be4) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2159   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           62        62           
  Lines         6829      6830    +1     
  Branches      1098      1098           
=========================================
+ Hits          6829      6830    +1     

Impacted Files	Coverage Δ
falcon/testing/helpers.py	100.00% <100.00%> (ø)

[liborjelinek]

Thank you for merging. I did it primarily for myself :-) My API heavily relies on correct %2F handling. When will it be approximately released in the PyPI?