Fix handling of float and double positive infinity values.

Npgsql doesn't quote float and double values when sending to server.
But special PositiveInfinite and NegativeInfinite values need to be quoted.

Fix #337
Change code to be project style conformant.
Improved Nan, PositiveInifinity and NegativeInfinity checking. Add
tests.

Thanks @roji for heads about about the IsNaN, IsPositiveInfinity and
IsNegativeInfinity methods.

Add tests for double and float array handing with PositiveInfinity and
NegativeInfinity.
Merge pull request #339 from franciscojunior/337-fix-float-double-infinite-value

Fix float double infinite value