npgsql - Npgsql2: Fixed problem where null parameters were being sent

Log Message:
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Fixed problem where null parameters were being sent with casts when they weren't needed and so causing problems. Added testcase for this problem. Thanks Andrus Moor for testcase and heads up.

Also added testcase for numeric float support with different cultureinfo.

Modified Files:
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Npgsql2/testsuite/noninteractive/NUnit20:
CommandTests.cs (r1.25 -> r1.26)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/npgsql/Npgsql2/testsuite/noninteractive/NUnit20/CommandTests.cs.diff?r1=1.25&r2=1.26)
Npgsql2/src/Npgsql:
NpgsqlParameter.cs (r1.23 -> r1.24)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/npgsql/Npgsql2/src/Npgsql/NpgsqlParameter.cs.diff?r1=1.23&r2=1.24)