Mounted router fall back to parent router when route is not found

[schloerke]

A different solution to #881

Situation:

• Static handler is mounted at /;
• Route /b/z is mounted at /a
• Route /c is mounted at /a/b

Goal: Request /a/b/c

Current behavior:

• The route of /a/b/c matches against mount location of /.
• The static file handler attempts to handle /a/b/c.
  • No file is found, a 404 is returned.

New behavior:

• The route of /a/b/c matches against mount location of /.
• The static file handler attempts to handle /a/b/c
  • No file is found, route is forwarded
  • Mount's $route() is marked as routeNotFound()
• The route of /a/b/c matches against mount location /a
  • The route is trimmed to /b/c
  • The route /b/c does not match any routes
  • Mount's $route() is marked as routeNotFound()
• The route of /a/b/c matches against mount location /a/b
  • The route is trimmed to /c
  • The route /c matches route location /c
  • Result of /c is returned

This would allow for #882 to not be merged, keep the docs as the last mount location. This benefits the user in that their content will always have preference over anything mounted in /__docs__/

PR task list:

• Should this behavior be opt in? Opt out?
• How should the user opt in / out?
  • $set_fallback(fallback = TRUE)?
  • $mount(path, router, fallback = TRUE)?
• Append mounted router, do not overwrite existing router
• Update NEWS
• Add tests
• Update documentation with devtools::document()

[jcheng5]

I suggest removing the IS_MOUNT tracking variable (tricky to make this work with async responses anyway) and instead have the route() method (which is recursive) never do 404 handling; instead, leave that to serve() (which is not recursive).

[jcheng5]

As written, you can choose to either have 1) 405 and missing-trailing-slash automatically handled for you, or 2) a custom top-level 404 handler, but you cannot have both.

[schloerke]

Going to wait on this PR for a bit as the changes are fairly drastic compared to current functionality