Fixed JSONP format (added semicolon)

[gabolaev]

According to wiki, JSONP must have a semicolon at the end of javascript expression.

callback_func({"foo": "bar", "baz": 42}); <-

[codecov]

Codecov Report

Merging #2007 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2007   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          40       40           
  Lines        2225     2225           
=======================================
  Hits         2196     2196           
  Misses         16       16           
  Partials       13       13

Impacted Files	Coverage Δ
render/json.go	87.34% <100%> (ø)	⬆️

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