fix: made trailing slash optional for namespaced routes

[ulken]

When using withNamespace(), requesting the "index" route of the namespace requires a trailing slash to match. The namespace example at the bottom of the readme does not work without appending a trailing slash to the url.

Example code:

const apiRoute = withNamespace('/api')

const routes = router(
  apiRoute(get('/', () => 'My api route'))
)

// Start server...

request(`${SERVER_URL}/api`)  <-- Does NOT work
request(`${SERVER_URL}/api/`) <-- Works

This PR makes the trailing slash optional for namespaced routes (in par with non-namespaced index routes). Note: It does not make trailing slashes optional for all routes, only for index routes.

[coveralls]

Coverage remained the same at 100.0% when pulling 30d562b on ulken:master into f959b7f on pedronauck:master.

[ulken]

An alternative approach would be to change the default to only match without trailing slash. If someone wants to be strict and require trailing slash they can always use a custom UrlPattern. Makes more sense to me at least.

Some might be concerned about SEO and duplicate content leaving it optional. Just putting it out there.