fix panic when []OptionFunc contains nil (#263)

NewRequest() can potentially be called with a
nil value present in []OptionFunc. For example
this code would trigger a panic prior to this
commit:

  _, _, err := git.Users.ModifyUser(event.UserId, options, nil)

gitlab.go

@@ -356,6 +356,10 @@ func (c *Client) NewRequest(method, path string, opt interface{}, options []Opti
 	}
 
 	for _, fn := range options {
+		if fn == nil {
+			continue
+		}
+
 		if err := fn(req); err != nil {
 			return nil, err
 		}