fix errcheck warnings (#1739)

binding/form.go

@@ -20,7 +20,11 @@ func (formBinding) Bind(req *http.Request, obj interface{}) error {
 	if err := req.ParseForm(); err != nil {
 		return err
 	}
-	req.ParseMultipartForm(defaultMemory)
+	if err := req.ParseMultipartForm(defaultMemory); err != nil {
+		if err != http.ErrNotMultipart {
+			return err
+		}
+	}
 	if err := mapForm(obj, req.Form); err != nil {
 		return err
 	}