Fix #217 double NaN regression

Fix #217 
After commit 854862aeb4ba2c7f6d6cd321baeddbddbeca74fd Npgsql wasn't adding quotes to float4 and float8 values anymore.
But according to documentation, NaN values need to be quoted. 

This patch reverts quote handling of float4 and float8 values.
I think it would be better to quote selectively in the conversion function.  It would be a tad more efficient, and of course you would usually not quote at all.