Don't overwrite existing Content-Type headers if not explicitly requested.
Don't overwrite existing Content-Type headers if not explicitly requested.

Fixes HTTPServerResponse.writeJsonBody to only set the content type header, if this was either explicitly requested, or if none was set. See #1601.

rejectedsoftware/vibe.d,chalucha/vibe.d,etcimon/vibe.d,etcimon/vibe.d,gedaiu/vibe.d,DBankov/vibe.d,Geod24/vibe.d