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.go

@@ -414,7 +414,6 @@ func (c *Context) PostFormArray(key string) []string {
 // a boolean value whether at least one value exists for the given key.
 func (c *Context) GetPostFormArray(key string) ([]string, bool) {
 	req := c.Request
-	req.ParseForm()
 	req.ParseMultipartForm(c.engine.MaxMultipartMemory)
 	if values := req.PostForm[key]; len(values) > 0 {
 		return values, true
@@ -437,7 +436,6 @@ func (c *Context) PostFormMap(key string) map[string]string {
 // whether at least one value exists for the given key.
 func (c *Context) GetPostFormMap(key string) (map[string]string, bool) {
 	req := c.Request
-	req.ParseForm()
 	req.ParseMultipartForm(c.engine.MaxMultipartMemory)
 	dicts, exist := c.get(req.PostForm, key)
 
@@ -465,6 +463,11 @@ func (c *Context) get(m map[string][]string, key string) (map[string]string, boo
 
 // FormFile returns the first file for the provided form key.
 func (c *Context) FormFile(name string) (*multipart.FileHeader, error) {
+	if c.Request.MultipartForm == nil {
+		if err := c.Request.ParseMultipartForm(c.engine.MaxMultipartMemory); err != nil {
+			return nil, err
+		}
+	}
 	_, fh, err := c.Request.FormFile(name)
 	return fh, err
 }