API: Treat URLs with and without trailing slash as identical

Currently, any API calls without trailing `/` character returns default "Flagsmith API" response. This behavior is quite unusual in API world as its recommended to primarily accept API requests without trailing slash.

To quote a few articles:

> A trailing forward slash (/) should not be included in URIs. This is one the most important rules to follow as the last character within a URI’s path, a forward slash (/) adds no semantic value and may cause confusion. REST API’s should not expect a trailing slash and should not include them in the links that they provide to clients.

> As the last character within a URI’s path, a forward slash (/) adds no semantic value and may confuse. It’s better to drop it from the URI.

It would be great to treat requests with and without trailing slash as identical.