* refactor(form_mapping.go): mapping ptr, struct and map
* fix#1672 correct work with ptr - not create value if field is not set
* avoid allocations on strings.Split() - change to strings.Index()
* fix#610 tag value "-" is mean ignoring field
* struct fields mapped like json.Unmarshal
* map fields mapped like json.Unmarshal
* fix after @thinkerou review
Dmitry Kutakov