Onf of the sides can be nil and it will raise a Conversion error

Fix an issue where Journey was failing to clear the named routes hash when the
routes were reloaded and since it doesn't overwrite existing routes then if a
route changed but wasn't renamed it kept the old definition. This was being
masked by the optimised url helpers so it only became apparent when passing an
options hash to the url helper.

When generating an unnamed url (i.e. using `url_for` with an options
hash) we should skip anything other than standard Rails routes otherwise
it will match the first mounted application or redirect and generate a
url with query parameters rather than raising an error if the options
hash doesn't match any defined routes.

Fixes #8018

Previously when app was mounted as following:

    class Foo
      def call(env)
        [200, {}, [env['PATH_INFO']]]
      end
    end

    RackMountRailsBug::Application.routes.draw do
      mount RackTest.new => "/foo"
    end

trailing slash was removed from PATH_INFO. For example requesting

    GET /foo/bar/

on routes defined above would result in a response containing "/foo/bar"
instead of "/foo/bar/".

This commit fixes the issue.

(closes #3215)

Adding a boolean route constraint checks for presence/absence of request property

It'd be a nice convention to mark the unused variables like this, now that Ruby 2 will issue no warnings for such vars being unused.

Rather than trying to use gsub to remove the optional route segments,
which will fail with nested optional segments, use a custom visitor
class that returns a empty string for group nodes.

Closes #9524

This commit changes route defaults so that explicit defaults are no
longer required where the key is not part of the path. For example:

  resources :posts, bucket_type: 'posts'

will be required whenever constructing the url from a hash such as a
functional test or using url_for directly. However using the explicit
form alters the behavior so it's not required:

  resources :projects, defaults: { bucket_type: 'projects' }

This changes existing behavior slightly in that any routes which
only differ in their defaults will match the first route rather
than the closest match.

Closes #8814

This now allows the use of arrays like this:

  get '/foo/:action', to: 'foo', constraints: { subdomain: %w[www admin] }

or constraints where the request method returns an Fixnum like this:

  get '/foo', to: 'foo#index', constraints: { port: 8080 }

Note that this only applies to constraints on the request - path
constraints still need to be specified as Regexps as the various
constraints are compiled into a single Regexp.

Now that Journey has been integrated into ActionDispatch we can raise
the exception ActionController::UrlGenerationError directly rather than
raising the internal Journey::Router::RoutingError and then have
ActionDispatch::Routing::RouteSet#generate re-raise the exception.

* prefer do-end for multiline blocks
* prefer or-equals over returns with checks

[ci skip]

Move the Journey code underneath the ActionDispatch namespace so
that we don't pollute the global namespace with names that may
be used for models.

Fixes rails/journey#49.