webio_serve(::Mux.App)

[cstjean]

The docstring says:

  webio_serve(app, port=8000)

  Serve a Mux app which might return a WebIO node.

but, frustratingly, it does not currently accept a Mux.app. This PR trivially fixes this issue.

[codecov]

Codecov Report

Merging #379 into master will decrease coverage by 0.1%.
The diff coverage is 0%.

@@           Coverage Diff            @@
##           master   #379      +/-   ##
========================================
- Coverage    62.1%    62%   -0.11%     
========================================
  Files          16     16              
  Lines         599    600       +1     
========================================
  Hits          372    372              
- Misses        227    228       +1

Impacted Files	Coverage Δ
src/providers/mux.jl	9.52% <0%> (-0.48%)	⬇️

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[shashi]

Thanks!