$wgDBmwschema used in place of $wgDBprefix when PostgreSQL is used

[tetsuo13]

The $wgDBprefix key in MediaWiki's settings is used to denote the table prefix otherwise $wgDBmwschema is used in its place when PostgreSQL is used. The application should be aware of this.

[tetsuo13]

Sort of discovered in #25 when reviewing the documentation on $wgDBprefix.