Falsey tags are being dropped from metrics #321

Hey all!

Ran into a problem where having a tag that looks like { some_tag: false } would result in the tag being dropped. This appears to be occurring because of the following line: https://github.com/DataDog/dogstatsd-ruby/blob/master/lib/datadog/statsd/serialization/tag_serializer.rb#L62

The output over the wire looks like:

some_counter:1|c when called with a hash that looks like { some_tag: false }

The expected output would be:

some_counter:1|c|#some_tag:false when the tag hash looks like { some_tag: false }

This isn't a breaking change, but the behavior resulted in unexpected results.