npgsql - Npgsql2: Added support to not add type casts with prepared

Log Message:
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Added support to not add type casts with prepared statements. Previous patch only added this capability for non prepared queries. Now, types without casts have the type written as "unknown" so type inference is done by server backend.

Modified Files:
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Npgsql2/src/Npgsql:
NpgsqlCommand.cs (r1.23 -> r1.24)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/npgsql/Npgsql2/src/Npgsql/NpgsqlCommand.cs.diff?r1=1.23&r2=1.24)