fix: Remove the trailing slash for the URI

When navigating on the Flagship app, we can pass in the
securiseUri method with  window['cozy']?.isSecureProtocol
=== true.

Result: we pass to the toString() method, and then we have
a trailing slash.

And since this uri is used when called by client.options.uri
some apps can concat that with other string without checking
if there is a trailing slash or not.

Apps should not do that, but since we have two different
behavior between the flagship and the no flagship app, we fix
that here.