Fix 404 issue

[jakubnowicki]

Fix 404 page not working when a non-valid page is opened as the first

Closes #124

Test application:

#Shiny Router 0.3.0 ----------
options(shiny.port = 3333)
library(shiny.router)
library(shiny)

router <- router_ui(
  route("/", shiny::tags$div(shiny::tags$span("Hello world"))),
  route("main", shiny::tags$div(h1("Main page"), p("Lorem ipsum."))),
  page_404 = page404(tags$div(tags$h1("This is the 404 Page")))
)

shinyApp(
  router,
  function(input, output, session) {router_server()}
)

# 1. Visit http://localhost:3333/#!/non-existing-page directly (without ever visiting homepage) from your browser
# 2. Visit http://localhost:3333/#!/ first and then visit http://localhost:3333/#!/non-existing-page

[Gotfrid]

Works as expected in the provided example snippet. Also, tested with a scaffolded Rhino app - works as well 💯