Fix text plain decoding (#824)

* test if the request with "text/plain" content type is parsed correctly

* add information on what Content-Type has to be used

this is only a cosmetic change

* use a CustomRequestDecoder to handle properly the misbehaving requests of the Web Editor
The requests sent uses `text/plain` Content-Type when they should be using `application/json`
This commit restores the old behavior, using always a json Decoder

Author: umbynos

design/design.go

@@ -24,5 +24,7 @@ var _ = API("arduino-create-agent", func() {
 	such as detecting which boards are connected and upload sketches on them.`)
 	HTTP(func() {
 		Path("/v2")
+		Consumes("application/json")
+		Consumes("plain/text")
 	})
 })