support mounting routes without a trailing slash

[mmerickel]

This PR replaces #1639. Closes #406.

The gist is this snippet that was added to the docs.

from pyramid.config import Configurator

def users_include(config):
    config.add_route('show_users', '')

def main(global_config, **settings):
    config = Configurator()
    config.include(users_include, route_prefix='/users')

The above configuration will match ``/users`` instead of ``/users/`` if a slash
had been supplied to the :meth:`pyramid.config.Configurator.add_route` function.

This is backward-incompatible for anyone mounting routes and using an empty string.

Alternative approaches that could maintain compatibility:

1. Support a sentinel value as the pattern. For example:

from pyramid.config import NO_APPEND_SLASH

def users_include(config):
    config.add_route('show_users', NO_APPEND_SLASH)

This is somewhat cumbersome.

2. Add an option to add_route that a user could opt into.

def users_include(config):
    config.add_route('show_users', '', route_prefix_append_slash=False)

I'm open to something like this and am curious what people think. Pyramid has a somewhat ingrained history of supporting '' == '/' and this makes them different, but only in a specific use case. When used from main or without a route_prefix, they are still equivalent and result in a trailing slash. I'm not convinced it's worth the incompatibility.

ping @nek4life

[stevepiercy]

Are you talking about only prefix-based routes, only routes without prefixes, or both?

I define routes with leading and trailing slashes to avoid any guesswork, so I don't have an opinion about this feature.

[mmerickel]

closing in favor of #3420