VOTable MIME type content parameter

[mbtaylor]

This is a marker issue for an item relating to the VOTable standard, see ivoa-std/VOTable#26.

Section 3.1 and other places in the DataLink document indicate use of the MIME type application/x-votable+xml;content=datalink. At time of writing, no such content parameter is defined in VOTable, so this is at least questionable practice.

This issue is here as a reminder that there should be some resolution of this discrepancy before DataLink 1.1 is endorsed.