fix produces with type parameter

[martian4202]

A path object can specify produces with an array of MIME types, as shown following:

/path:
    produces:
          - text/plain; charset=utf-8
    ....

In the latest release, the MIME-type parameters(charset=utf-8 in this case) is normalized and ignored and we get a map from the normalized MIME type to the runtime.Producer. However, when determining what runtime.Producer to use to response to the client, it uses the unnormalized MIME type.

var format string
format, r = c.ResponseFormat(r, offers)
rw.Header().Set(runtime.HeaderContentType, format)

if resp, ok := data.(Responder); ok {
	producers := route.Producers
	prod, ok := producers[format]

[codecov]

Codecov Report

Merging #251 (1c6d707) into master (5bcfe65) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
- Coverage   80.18%   80.13%   -0.06%     
==========================================
  Files          42       42              
  Lines        3189     3191       +2     
==========================================
  Hits         2557     2557              
- Misses        519      521       +2     
  Partials      113      113              

Impacted Files	Coverage Δ
middleware/context.go	73.55% <0.00%> (-0.41%)	⬇️

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