router: handling file not found in StaticFS() to use Router's NoRoute

[oantoro]

Refs #1182

[oantoro]

CI error due to fetching dependencies timed out in Go version master.

[se3000]

@okyantoro if you rebase and push, it should kick off a new travis build.

[codecov]

Codecov Report

Merging #1220 into master will decrease coverage by 0.25%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master    #1220      +/-   ##
==========================================
- Coverage   99.27%   99.01%   -0.26%     
==========================================
  Files          39       39              
  Lines        1931     1937       +6     
==========================================
+ Hits         1917     1918       +1     
- Misses         10       14       +4     
- Partials        4        5       +1

Impacted Files	Coverage Δ
routergroup.go	94.44% <16.66%> (-5.56%)	⬇️

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[thinkerou]

@okyantoro please add unit/integration test case, thanks!