* feat(binding): support custom struct tag
Add function `binding.MapFormWithTag` (#2719)
* doc: add 'bind form-data with custom struct tag'
Add 'Bind form-data request with custom struct and custom tag' section (#2719)
* test(binding): add test for MapFromWithTag
* move tests of mapping to separate test file
make 100% coverage of form_mapping.go from form_mapping_test.go file
* fix tests for go 1.6
go 1.6 doesn't support `t.Run(...)` subtests
