Add nested oneof test

[momom-i]

References to other Issues or PRs

#753

Have you read the Contributing Guidelines?

Yes

Brief description of what is fixed or changed

When the issue was created, grpc_gateway version seemed v1 and the problem mentioned in it is fixed in v2.
But I wrote test that checked nested oneof value case.

Other comments

[codecov-commenter]

Codecov Report

Merging #2323 (ca0d224) into master (77176e3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2323   +/-   ##
=======================================
  Coverage   59.64%   59.64%           
=======================================
  Files          35       35           
  Lines        4500     4500           
=======================================
  Hits         2684     2684           
  Misses       1525     1525           
  Partials      291      291           

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[johanbrandhorst]

This is really great, thanks a lot for this test!