[VE][html5] Bad value Content-Type for attribute http-equiv on XHTML element meta

The following error does not make sense

Error Line 8, Column 81: Bad value Content-Type for attribute http-equiv on
XHTML element meta.

…eta http-equiv="Content-Type" content="application/xhtml+xml;
charset=utf-8" />

These documents are being served with the actual content type of
application/xhtml+xml but for archival purposes it makes sense to keep the
meta content type representing this state.  Because of this error, the only
way to validate the document may be to remove the content type from but in
doing so you lose the mime type on most current operating systems since few
filesystems associate files with mime types.