Always use period for decimal point when converting double to string

[elijahandrews]

Right now, conversion from double to string executed by NonBlockingStatsDClient.FORMAT depends on the locale of the user. If the user's locale is one where a comma is used as a decimal point, the Datadog Agent will not be able to parse a metric with a double value.

[elijahandrews]

Thanks!

[charles-dyfis-net]

Fix released in 2.0.8; thank you for the report, and @kaufda for the fix.