Added InvariantCulture to NpgsqlCopySerializer

PostgreSQL currently only supports SQL notation for decimal point (which
is the same as InvariantCulture)
Added static readonly CultureInfo field for use in string.Format to
always use dot as decimal point.
Add test for #92
Merge pull request #1 from franciscojunior/add-test-for-pr92

Add test for pr92
Merge pull request #92 from djsubzero/master

Added InvariantCulture to NpgsqlCopySerializer