update_oai-pmh fix URL handling for endpoints with no trailing slash

Motivation:

A bug was introduced with commit aee25f9f where OAI-PMH URLs without a
trailing slash have one added; for example, the Identify request
targeting PSI changes from `https://doi.psi.ch/oaipmh/oai?verb=Identify`
to `https://doi.psi.ch/oaipmh/oai/?verb=Identify`.

For some endpoints, this distinction is important, resulting in the
requests failing.

Modification:

Fix regression so that the correct URL is used.

Result:

The PSI endpoint is now shown (correctly) to be working.