BUG/MEDIUM: http-htx: Don't use data from HTX message to update authority

BUG/MEDIUM: http-htx: Don't use data from HTX message to update authority

When a host header value is updated, the authority can also be updated
accordingly. When it is performed, we must not use the new host header value
from the HTX message. Instead we must use the data passed as argument. It is
unexpected but the host header can have several comma-separated values.
Using the full header value can lead to unexpected result.

Note: having multiple comma-separated values for the host header should not
      be supported. The comma should be part of the host value. But it is
      quite ambiguous. This will be fixed in another commit.

This patch must be backported to all stable versions.