support assetserving of files with spaces

[hhaensel]

Currently assetserving fails when files contain spaces or other url-forbidden characters.

I propose to modify assetserve() in order to solve that issue.

[codecov-commenter]

Codecov Report

Merging #105 into master will decrease coverage by 3.38%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
- Coverage   58.21%   54.83%   -3.39%     
==========================================
  Files           7        7              
  Lines         146      155       +9     
==========================================
  Hits           85       85              
- Misses         61       70       +9     

Impacted Files	Coverage Δ
src/examples/files.jl	16.12% <0.00%> (-2.39%)	:arrow_down:
src/server.jl	70.83% <0.00%> (-3.08%)	:arrow_down:
src/websockets_integration.jl	0.00% <0.00%> (ø)

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[cmcaine]

Thanks! Sorry it took so long to get to this :)