Update Statsd tag serializer to allow falsy values

[joseph-hughes]

Please, see #290 for all details.

PR description at the time of opening this issue:

Problem

If we provide a tag which has a boolean value, we are unable to properly track the false case, since the "true" string appears with the tag name, but the "false" string does not show up. This is because of this logic, which checks for tag value presence:

dogstatsd-ruby/lib/datadog/statsd/serialization/tag_serializer.rb

Lines 58 to 66 in 517b830

	def to_tags_list(tags)
	case tags
	when Hash
	tags.map do |name, value|
	if value
	escape_tag_content("#{name}:#{value}")
	else
	escape_tag_content(name)
	end

true is evaluated to present, so we would add that tag name and value; however, false is evaluated to not present, so we would only add that tag name (not the value).

Solution

First, check for nil explicitly then also check for nil after converting to a string.

This does result in other values being deemed "present" now which were not previously though, such as [] and {}. Is this a problem though?