Revoke refresh token with a HTTP POST instead of a HTTP GET call

[wmathurin]

Manually tested by:

• stepping through debugger on the client
• watching the session management page in the browser at the same time to see the oauth2 session disappear

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.57%. Comparing base (8f25ca2) to head (20d920a).
Report is 32 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #3889   +/-   ##
=======================================
  Coverage   62.56%   62.57%           
=======================================
  Files         247      247           
  Lines       22313    22317    +4     
=======================================
+ Hits        13961    13965    +4     
  Misses       8352     8352           

Components	Coverage Δ
Analytics	70.78% <ø> (ø)
Common	69.48% <ø> (ø)
Core	52.44% <100.00%> (+0.01%)	⬆️
SmartStore	73.66% <ø> (ø)
MobileSync	87.66% <ø> (ø)

Files with missing lines	Coverage Δ
...KCore/SalesforceSDKCore/Classes/Util/SFSDKOAuth2.m	55.20% <100.00%> (+0.52%)	⬆️

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[wmathurin]

Only one test failure in WebViewStateManagerTests which passes locally. It does not seem related to this change.