Ensure SQL Server default schema name not interpreted as dot-separated tokens

Ensure SQL Server default schema name not interpreted as dot-separated tokens

Fixed an issue in the :meth:`.Engine.table_names` method where it would
feed the dialect's default schema name back into the dialect level table
function, which in the case of SQL Server would interpret it as a
dot-tokenized schema name as viewed by the mssql dialect, which would
cause the method to fail in the case where the database username actually
had a dot inside of it.  In 1.3, this method is still used by the
:meth:`.MetaData.reflect` function so is a prominent codepath. In 1.4,
which is the current master development branch, this issue doesn't exist,
both because :meth:`.MetaData.reflect` isn't using this method nor does the
method pass the default schema name explicitly.  The fix nonetheless
guards against the default server name value returned by the dialect from
being interpreted as dot-tokenized name under any circumstances by
wrapping it in quoted_name().

Fixes: #4923
Change-Id: I821bd38ed89b767eaca0bdffee7f8ba3baf82560