proto-gen-swagger: provide default description for HTTP 200 responses

[johnchildren]

In the swagger specification it says the following about description fields:

Field Name	Type	Description
description	string	REQUIRED. A short description of the response. CommonMark syntax MAY be used for rich text representation.

https://swagger.io/specification/#responseObject

Currently proto-gen-swagger will generate an empty string for this field on a 200 response, but I am unsure if this is enough to meet the specification. This has some ramifications as at least one of the generators in swagger-codegen or openapi-codegen, rust-server will not compile if the description is set to an empty string without manual intervention.

It seems to be this line which is responsible, so it may just be a matter of setting a default value, though I am not sure what this value should actually be.

https://github.com/grpc-ecosystem/grpc-gateway/blob/master/protoc-gen-swagger/genswagger/template.go#L697

[johanbrandhorst]

Thanks for submitting this issue @johnchildren! This does indeed seem to be in breach of the spec. What can I do to help you get a pull request in to fix this? I think it should be okay to introduce this backwards-incompatibility since any comments or explicit description will override it. How about just A successful response?

[johnchildren]

Have created a pull request for the change, but I am unsure how best to provide automated tests. I also interested to see how the default value looks when used with streaming responses.

[johanbrandhorst]

Closed with #767