Fix gen-swagger to support more well known types

[shouichi]

Some well know types are not handled correctly. Given we have the following proto message.

// proto
message User {
  google.protobuf.StringValue name = 1;
}

name is output as an object.

// swagger.json
"name": {
  "$ref": "#/definitions/protobufStringValue"
}

It should be output as a primitive.

// swagger.json
"name": {
  "type": "string"
}

This PR adds support for the following well know types.

• StringValue
• Int32Value
• Int64Value
• FloatValue
• DoubleValue
• BoolValue

[achew22]

This looks like a great start! I'm excited to merge this. Could you add some tests to verify schemaOfField does the right thing for these new well known types? It looks like we are totally missing coverage on that method right now.

https://github.com/shouichi/grpc-gateway/blob/31e65b13c82edfcf1c78926c59fa829b7ed14841/protoc-gen-swagger/genswagger/template.go#L250

[shouichi]

Sure, thanks for your advice!

[roadrunner]

+1

[shouichi]

Added test for schemaOfField. I'm not sure if I set up descriptor registry correctly. Feel free to comment if I'm doing something wrong.

[achew22]

Sorry this took so long. Looking through the docs a bit I think this is correct. Merging. Thanks for your contribution!