404 when trying to ->authorize() against Akkoma

I don't know if this affects Mastodon servers or not but I had to adjust line 174 in Mastodon::Client to include a / before oauth so it would trigger the check on line 205 of Mastodon::Role::UserAgent.

The line becomes: my $response = $self->post( '/oauth/token' => $data );

Before it was trying to hit /api/v1/oauth/token and this isn't valid on Akkoma instances, and Akkoma's docs suggest that it's not correct on Mastodon instances either. Whether it's valid or not remains to be seen.

Thanks for this code though!