protoc-gen-swagger: should use json_name values for generating properties in swagger doc

[adithya66]

The gRPC-Gateway project is maintained by volunteers in their spare time. Please follow these troubleshooting steps before submitting an issue.

• Check if your issue has already been reported (https://github.com/grpc-ecosystem/grpc-gateway/issues).
• Update your protoc to the latest version.
• Update your copy of the grpc-gateway library to the latest version from github:

  go get -u github.com/grpc-ecosystem/grpc-gateway

• Delete the protoc-gen-grpc-gateway and protoc-gen-swagger binary from your PATH,
  and reinstall the latest versions:

  go get -u github.com/grpc-ecosystem/grpc-gateway/protoc-gen-grpc-gateway
  go get -u github.com/grpc-ecosystem/grpc-gateway/protoc-gen-swagger

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