Fixed double padding in Group.File, Group.Add (#1534)

Group.File was padding with g.prefix even though it would later call Group.Add which padded with prefix again - for a total of two times

Author: Ori Shoshan

group.go

@@ -109,7 +109,7 @@ func (g *Group) Static(prefix, root string) {
 
 // File implements `Echo#File()` for sub-routes within the Group.
 func (g *Group) File(path, file string) {
-g.file(g.prefix+path, file, g.GET)
+g.file(path, file, g.GET)
 }
 
 // Add implements `Echo#Add()` for sub-routes within the Group.

group_test.go

@@ -1,7 +1,9 @@
 package echo
 
 import (
+"io/ioutil"
 "net/http"
+"net/http/httptest"
 "testing"
 
 "github.com/stretchr/testify/assert"
@@ -26,6 +28,19 @@ func TestGroup(t *testing.T) {
 g.File("/walle", "_fixture/images//walle.png")
 }
 
+func TestGroupFile(t *testing.T) {
+e := New()
+g := e.Group("/group")
+g.File("/walle", "_fixture/images/walle.png")
+expectedData, err := ioutil.ReadFile("_fixture/images/walle.png")
+assert.Nil(t, err)
+req := httptest.NewRequest(http.MethodGet, "/group/walle", nil)
+rec := httptest.NewRecorder()
+e.ServeHTTP(rec, req)
+assert.Equal(t, http.StatusOK, rec.Code)
+assert.Equal(t, expectedData, rec.Body.Bytes())
+}
+
 func TestGroupRouteMiddleware(t *testing.T) {
 // Ensure middleware slices are not re-used
 e := New()