fix produces with type parameter

middleware/context.go

@@ -498,7 +498,9 @@ func (c *Context) Respond(rw http.ResponseWriter, r *http.Request, produces []st
 
 	if resp, ok := data.(Responder); ok {
 		producers := route.Producers
-		prod, ok := producers[format]
+		// producers contains keys with normalized format, if a format has MIME type parameter such as `text/plain; charset=utf-8`
+		// then you must provide `text/plain` to get the correct producer. HOWEVER, format here is not normalized.
+		prod, ok := producers[normalizeOffer(format)]
 		if !ok {
 			prods := c.api.ProducersFor(normalizeOffers([]string{c.api.DefaultProduces()}))
 			pr, ok := prods[c.api.DefaultProduces()]