add support time.Duration on mapping

[vkd]

var s struct {
    Timeout time.Duration `form:"timeout"`
}
req := formPostRequest("", "timeout=5s")
Form.Bind(req, &s)

[codecov]

Codecov Report

Merging #1794 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1794      +/-   ##
==========================================
+ Coverage   98.62%   98.62%   +<.01%     
==========================================
  Files          41       41              
  Lines        2103     2112       +9     
==========================================
+ Hits         2074     2083       +9     
  Misses         18       18              
  Partials       11       11

Impacted Files	Coverage Δ
binding/form_mapping.go	100% <100%> (ø)	⬆️

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[thinkerou]

LGTM