Fixed error reassignment

[asardak]

Hello! I have noticed that DefaultHTTPErrorHandler always logs "message":"<nil>" for every error:

{"time":"2017-09-08T09:25:46.390299469Z","level":"ERROR","prefix":"echo","file":"api.go","line":"67","message":"<nil>"}

This PR fix it by shadowing original err variable in conditional block.

[codecov]

Codecov Report

Merging #1006 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1006      +/-   ##
==========================================
+ Coverage   76.52%   76.53%   +0.01%     
==========================================
  Files          26       26              
  Lines        2224     2225       +1     
==========================================
+ Hits         1702     1703       +1     
  Misses        400      400              
  Partials      122      122

Impacted Files	Coverage Δ
echo.go	85.62% <100%> (+0.04%)	⬆️

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[vishr]

@asardak Thanks for your contribution, I got it covered in a different commit.