Let users specify if/how None values are encoded

[nphilipp]

Fixes #125.

Changes proposed in this pull request:

• Adds an encode_none parameter to Query which lets users specify if at all, and if yes how a value of None should be sent to the service.

This changes behavior slightly: if a parameter is None (per its default value or because the user set it), previously it was encoded as ...?name=None. To retain this behavior, a user would have to specify Query(..., encode_none="None").

Attention: @prkumar

[codecov]

Codecov Report

Merging #126 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #126   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          25     25           
  Lines        1419   1425    +6     
  Branches      134    136    +2     
=====================================
+ Hits         1419   1425    +6

Impacted Files	Coverage Δ
uplink/arguments.py	100% <100%> (ø)	⬆️

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[prkumar]

Looks good! Also, thanks for including documentation with your changes🥇