Fix getMuxWSUrl for location.pathname other than "/"

[etpinard]

fixes #382

---

Manually tested with the following:

• in ~/.julia/dev/WebIO/ :

pkg> test WebIO

this builds the JS bundles under deps/bundles/ (and also runs the tests locally)

• in some tmp folder

pkg> activate .
pkg> add Interact Mux
pkg> dev WebIO
pkg> st
  [c601a237] Interact v0.10.3
  [a975b10e] Mux v0.7.1
  [0f1e0344] WebIO v0.8.13 [`~/.julia/dev/WebIO`]

julia> using WebIO, Interact, Mux
   
julia> function gui()
           but = button("click me")
           on(but) do _
               println("was clicked")
           end
           but
       end
gui (generic function with 1 method)

julia> Mux.@app app = (Mux.defaults,
                       page("/", req->gui()),
                       page("/hallo", req->gui()),
                       page("/hallo/hello", req->gui()),
                       Mux.notfound())

julia> webio_serve(app, 8000)
Task (runnable) @0x00007f359a5adae0

where the button works on http://localhost:8000/, http://localhost:8000/hallo and http://localhost:8000/hallo/hello

---

Please let me know if you want me to (try to) add a tests for this. Thank you!

cc @cstjean

[shashi]

Looks good to me. cc @travigd

[twavv]

Travis-CI failures are unrelated. LGTM.