Honor x-b3 request headers about kong-plugin-zipkin

[kikito]

The traceId gets respected and passed on. The other header fields are span Id and parent span Id, which should really be set by Kong and not a third party (since Kong is the one creating the spans).

If someone is experiencing issues where a trace id is not being passed on, it is likely to happen somewhere else. For instance, in a network where Kong is used as a middleman between nodes, like so:

client --> Kong ---> A ---> Kong ---> B

It's possible that Service A is the one mangling the headers. The Kong node between A and B would add a new trace Id, and the two traces would appear unrelated.