[FIX] allow empty headers on DataFromReader

[vkd]

Fixed #2111

Now is possible to call c.DataFromReader without extra headers.

c.DataFromReader(http.StatusOK, 5, gin.MIMEHTML, strings.NewReader("1234\n"), nil)

Thanks: @nikandfor

[codecov]

Codecov Report

Merging #2121 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2121      +/-   ##
==========================================
+ Coverage   98.61%   98.61%   +<.01%     
==========================================
  Files          40       40              
  Lines        2236     2238       +2     
==========================================
+ Hits         2205     2207       +2     
  Misses         16       16              
  Partials       15       15

Impacted Files	Coverage Δ
render/reader.go	100% <100%> (ø)	⬆️

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