Set default time format in form binding

[lokhman]

To make things working in line with encoding/json decoder, I'd suggest to set default time format as time.RFC3339.

type input struct {
	Field time.Time `form:"field" json:"time"`
}

This will make values bound in the same way as if you are using structures as above and different bindings depending on the request type.

[codecov]

Codecov Report

Merging #1487 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1487   +/-   ##
=======================================
  Coverage   99.15%   99.15%           
=======================================
  Files          36       36           
  Lines        1894     1894           
=======================================
  Hits         1878     1878           
  Misses         13       13           
  Partials        3        3

Impacted Files	Coverage Δ
binding/form_mapping.go	98.54% <100%> (ø)	⬆️

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[thinkerou]

@lokhman thanks, LGTM