Format issues

[jcutrell]

It seems as though the format specification for Zipkin does not allow for null values (for example, for the parent ID).

Similarly, it seems that tags are intended to be defined as strings only.

The encoder should probably filter out key-value pairs from the serialization where values are null, and tag values should probably be converted to strings in a safe manner.

I came upon these issues when trying to send Zipkin spans to a Jaeger backend, and then found the data model for Zipkin here: https://zipkin.io/zipkin-api/#/default/post_spans

Perhaps I am misunderstanding something here?