'type' should be 'integer' for int64

[koron]

According to OpenAPI 2.0 spec, "type" should be "integer" for int64, not "string"
cf: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#data-types

so current protoc-gen-swagger generates invalid swagger file when int64 is used.

This PR fix this problem, just replace "string" by "integer".

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[koron]

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[codecov-io]

Codecov Report

Merging #810 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #810   +/-   ##
======================================
  Coverage    53.3%   53.3%           
======================================
  Files          30      30           
  Lines        3371    3371           
======================================
  Hits         1797    1797           
  Misses       1399    1399           
  Partials      175     175

Impacted Files	Coverage Δ
protoc-gen-swagger/genswagger/template.go	38.52% <ø> (ø)	⬆️

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[johanbrandhorst]

This has been discussed before in #438, I don't think we will be changing this as it's fundamentally wrong when applied to JSON. See OAI/OpenAPI-Specification#704 for more discussion on this, but as the gRPC-Gateway works with JSON for swagger, we will not be changing this.

[koron]

I see very well. Thank you for explaing it. 🙇