In some scenarios prefix needs to be ignored when wrapping DOI

[Tom Najdek]

After #124, with the new link_anchors set to true, for the following CSL (copied from MLA):

<text variable="DOI" prefix="https://doi.org/"/>

The following HTML is produced:

https://doi.org/<a href="https://doi.org/VAR">https://doi.org/VAR</a>

https://doi.org/ part of the prefix shouldn't be used here as it already appears as part of the URL.

Related: #116