Fix proto3 JSON

[aaronc]

ref #6213

Adding support for Amino JSON to Any broke proto3 JSON (jsonpb). This PR fixes that and adds tests.

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[codecov]

Codecov Report

Merging #6345 into master will increase coverage by 0.00%.
The diff coverage is 73.17%.

@@           Coverage Diff           @@
##           master    #6345   +/-   ##
=======================================
  Coverage   55.66%   55.66%           
=======================================
  Files         448      448           
  Lines       26965    26983   +18     
=======================================
+ Hits        15009    15020   +11     
- Misses      10879    10884    +5     
- Partials     1077     1079    +2     

[fedekunze]

utACK

[alexanderbez]

ACK