Add tests for float4 and float8 NaN inserting.
Fix #217 when handling NaN for float4 and float8 datatypes.

After commit 854862aeb4ba2c7f6d6cd321baeddbddbeca74fd Npgsql wasn't
adding quotes around float4 and float8 values.

But according to documentation, NaN values need to be quoted.

Separated handling of float4 and float8 conversion routines.
Add support to handle NaN values inside arrays. Thanks @glenebob for heads up.

Add tests for Float array.
Backport #257 from master.