Add support for key-only request parameters

[JWCook]

Closes #499

[codecov-commenter]

Codecov Report

Merging #500 (483574f) into master (0185d02) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #500   +/-   ##
=======================================
  Coverage   98.75%   98.76%           
=======================================
  Files          18       18           
  Lines        1290     1293    +3     
  Branches      185      187    +2     
=======================================
+ Hits         1274     1277    +3     
  Misses          8        8           
  Partials        8        8           

Impacted Files	Coverage Δ
requests_cache/cache_keys.py	100.00% <100.00%> (ø)

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