Pass MaxMultipartMemory when FormFile is called (#1600)

When `gin.Context.FormFile("...")` is called the `engine.MaxMultipartMemory` is never used. This PR makes sure that the `MaxMultipartMemory` is passed and removes 2 calls to `http.Request.ParseForm` since they are called from `http.Request.ParseMultipartForm`

context_test.go

@@ -84,6 +84,19 @@ func TestContextFormFile(t *testing.T) {
 	assert.NoError(t, c.SaveUploadedFile(f, "test"))
 }
 
+func TestContextFormFileFailed(t *testing.T) {
+	buf := new(bytes.Buffer)
+	mw := multipart.NewWriter(buf)
+	mw.Close()
+	c, _ := CreateTestContext(httptest.NewRecorder())
+	c.Request, _ = http.NewRequest("POST", "/", nil)
+	c.Request.Header.Set("Content-Type", mw.FormDataContentType())
+	c.engine.MaxMultipartMemory = 8 << 20
+	f, err := c.FormFile("file")
+	assert.Error(t, err)
+	assert.Nil(t, f)
+}
+
 func TestContextMultipartForm(t *testing.T) {
 	buf := new(bytes.Buffer)
 	mw := multipart.NewWriter(buf)