Extension helper with fixed clauses shielding

[Suor]

I detected that clauses weren't copied properly in _extension(). This was cause of using instanceof check where really "subclass of" check should be.

I goed to explicit naming what to subclass though, since there are also aliases insertInto and deleteFrom, which better be still aliases, not distinct subclasses.

[Suor]

I also added a test for _extension() shielding.

[prust]

Looks good, @Suor, thanks! Merging.

[prust]

@Suor: I bumped the minor version (to 1.1.0), since these changes affect people using SQLBricks via an extension library (postgresql-bricks/sqlite-bricks), though I doubt there are very many people doing that right now.