modify Roa Composer:allow user to set contet-type

sdk/auth/roa_signature_composer.go

@@ -77,7 +77,9 @@ func completeROASignParams(request requests.AcsRequest, signer Signer, regionId
 	if request.GetFormParams() != nil && len(request.GetFormParams()) > 0 {
 		formString := utils.GetUrlFormedMap(request.GetFormParams())
 		request.SetContent([]byte(formString))
-		headerParams["Content-Type"] = requests.Form
+		if headerParams["Content-Type"] == "" {
+			headerParams["Content-Type"] = requests.Form
+		}
 	}
 	contentMD5 := utils.GetMD5Base64(request.GetContent())
 	headerParams["Content-MD5"] = contentMD5