fix: parseField skips not-exported (encoding/json-like)

[tcarreira]

Describe the PR
In order to match encoding/json behaviour for Marshaling struct into JSON, we should not parse non-exported fields (aka: fields starting with a lower letter)

Relation issue
#666

Additional context
With this commit, I had to apply a small fix to a test. I really think it's ok, but please, verify it.

[sdghchj]

So you dropped backward compatibility, though it's not a big trouble.

[sergeydobrodey]

This feature is very useful when structures are generated using new version of protoc-gen-go: https://blog.golang.org/protobuf-apiv2

Generated structures have technical fields without json:"-" annotation. Maybe you can add run flag if backward compatibility is obligatory?

[easonlin404]

Although causing non-backward compatibility, it makes sense that not-exported fields don't need to parse for marshaling struct into JSON.

[easonlin404]

What's your guys thought on it? @swaggo/swaggo-core-team

[codecov-io]

Codecov Report

Merging #675 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #675      +/-   ##
==========================================
+ Coverage   84.98%   85.00%   +0.01%     
==========================================
  Files           7        7              
  Lines        1805     1807       +2     
==========================================
+ Hits         1534     1536       +2     
  Misses        172      172              
  Partials       99       99              

Impacted Files	Coverage Δ
parser.go	79.79% <100.00%> (+0.04%)	⬆️

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