npgsql - Npgsql2: Fixed bug in skip.

Log Message:
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Fixed bug in skip. Thanks to gavin w for reporting the bug.

Modified Files:
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Npgsql2/src/Npgsql/SqlGenerators:
SqlBaseGenerator.cs (r1.17 -> r1.18)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/npgsql/Npgsql2/src/Npgsql/SqlGenerators/SqlBaseGenerator.cs.diff?r1=1.17&r2=1.18)
VisitedExpression.cs (r1.11 -> r1.12)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/npgsql/Npgsql2/src/Npgsql/SqlGenerators/VisitedExpression.cs.diff?r1=1.11&r2=1.12)