Add casting for numeric/float4/float8 type value

Add casting for numeric/float4/float8 type value
automatically to compensate the lack of automatic
conversion functionality of PostgreSQL server.
For example if there's a numeric type binding
   1.2567 --> 1.2567::numeric.
I hope this change would enable the use of numeric
type in MS-Access etc.

Thanks Hiroki Kataoka for his checking my code.