Remove Content-Type for empty responses

[databus23]

This is an attempt at fixing go-openapi/runtime#64.

Some clients trip over responses with empty bodies and Content-Type: application/json as they feed the body directly to a JSON parser. As a zero length string is not a valid JSON this causes a parse error.
As a prominent client jQuery barfs on this intentionally since 1.9: https://jquery.com/upgrade-guide/1.9/#jquery-ajax-returning-a-json-result-of-an-empty-string :(

Note that at the moment this change will cause empty responses to have Content-Type: text/plain set because its what go's http server does by default: golang/go#20784.
Hopefully this will change in later go versions.
Nonetheless this fixes pedantic clients as an empty string is valid text/plain

[codecov]

Codecov Report

Merging #1249 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1249   +/-   ##
=======================================
  Coverage   67.54%   67.54%           
=======================================
  Files          28       28           
  Lines        7812     7812           
=======================================
  Hits         5277     5277           
  Misses       2048     2048           
  Partials      487      487

Impacted Files	Coverage Δ
generator/bindata.go	60.32% <100%> (ø)	⬆️

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[databus23]

Just adding that golang/go#20784 was closed so the spurious text/plain content-type should be gone with go 1.10

[pheepi]

Unfortunately, this change causes unexpected behavior when the content type is previously set by WithContentType or SetContentType methods. The generated WriteResponse method might look like this:

// WriteResponse to the client
func (o *HeadOK) WriteResponse(rw http.ResponseWriter, producer runtime.Producer) {

	// response header Content-Type

	contentType := o.ContentType
	if contentType != "" {
		rw.Header().Set("Content-Type", contentType)
	}

	rw.Header().Del(runtime.HeaderContentType) //Remove Content-Type on empty responses

	rw.WriteHeader(200)
}

The content type is deleted even if it is wanted. I do not agree that empty response should not carry Content-type header from go-openapi/runtime#64, at least in case of HEAD requests, because it is one of often function.

[vincentvanderweele]

Agree with @pheepi. I'm running in the issue now that the Content-Type header is stripped from my HEAD responses and there is no way to work around this.

As far as I know, WriteResponse is scoped to a model, not an operation, so it cannot decide if this header should be there or not.

Should this change be reverted and the burden moved to the application developer to just not set the Content-Type header if it is undesired?