ContentType does not return charset when using the default encoder

[james-haan]

Expected this to return the appended charset,

Response(StatusOK, func() {
	ContentType("application/json; charset=utf-8")
})

but got this instead.

< HTTP/1.1 200 OK
< Content-Type: application/json

Found the code which was overriding the original content type to be the parsed mime type.
However I was curious if this was intended(by making the user to implement a custom encoder).

goa/http/encoding.go

Line 153 in 2fb1a45

SetContentType(w, mt)

I changed the code to ct which is provided from the dsl and returns the wanted charset.

< HTTP/1.1 200 OK
< Content-Type: application/json; charset=utf-8