npgsql - Npgsql2: Fix [#1011100] NpgsqlParameter and DBNull Value.

Log Message:
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Fix [#1011100] NpgsqlParameter and DBNull Value. Now NpgsqlParameter changes the dbtype accordingly to the value assigned to it unless the dbtype has been assigned explicitly.

Modified Files:
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Npgsql2/src/Npgsql:
NpgsqlParameter.cs (r1.27 -> r1.28)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/npgsql/Npgsql2/src/Npgsql/NpgsqlParameter.cs?r1=1.27&r2=1.28)