Add test cases for JSON resource marshaling

[sagar23sj]

Resources Covered :
- BranchCommit
- CommitsComparison
- CommitFile
- CommitStats
- RepositoryCommit

Helps With issue : #55

[codecov]

Codecov Report

Merging #2009 (105bf45) into master (9cd1df3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2009   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files         107      107           
  Lines        6897     6897           
=======================================
  Hits         6750     6750           
  Misses         81       81           
  Partials       66       66           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9cd1df3...105bf45. Read the comment docs.

[gmlewis]

Thank you, @sagar23sj !
LGTM.
Merging.