Added test cases for JSON resource marshaling

[Harikesh00]

Added test cases for DispatchRequestOptions, TransferRequest, SignaturesProtectedBranch, DismissalRestrictionsRequest of file github/repos.go

contribution to #55

[google-cla]

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[codecov]

Codecov Report

Merging #2519 (79e3452) into master (672fd67) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2519   +/-   ##
=======================================
  Coverage   97.97%   97.97%           
=======================================
  Files         123      123           
  Lines       10813    10813           
=======================================
  Hits        10594    10594           
  Misses        150      150           
  Partials       69       69           

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[Harikesh00]

closing because of cla issue
new PR raised for the same is #2520