Use verified schema name, not raw passed argument.

This should fix http://trac.osgeo.org/postgis/ticket/2708 where 

```sql
SELECT updategeometrysrid('public', 'test','geom_utm', 32630);
```

would alter the `enforce_srid`-constraint, but

```sql
SELECT updategeometrysrid('test','geom_utm', 32630);
``` 

wouldn't; even though

```sql
SELECT current_schema()::text;
``` 

would yield `'public'`.

The changes are untested, but the use of the verified schema name seems to be the correct way to check for the `enforce_srid`-constraint.