npgsql - Npgsql2: Fixed a typo which prevented DateTime.MaxValue to

Log Message:
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Fixed a typo which prevented DateTime.MaxValue to work on previous patch.
Thanks Andrus Moor for heads up.

Modified Files:
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Npgsql2/src/Npgsql:
NpgsqlParameter.cs (r1.19 -> r1.20)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/npgsql/Npgsql2/src/Npgsql/NpgsqlParameter.cs.diff?r1=1.19&r2=1.20)