Fixed servlet/client incompatibility if parentSpanId is not provided

[srapp]

Stumbled upon this bug when I was trying to instigate some tracing with some client code. The ParentSpanId is an optional parameter that JerseyClientTraceFilter serializes in the headers, either as a String representation of the provided Long, or the String "null". The ServletTraceFilter of a downstream service will attempt to deserialize this parameter, but it expects either a null value (as if the header was not provided) or a String representation of a long. It does not know how to handle the String "null".

My proposed change includes a ParentSpanId header only if a value is provided, and includes some unit tests around the JerseyClientTraceFilter and a compatibility test between the client and servlet filters.

[kristofa]

I didn't use or test the jersey integration myself. It is a contribution from another user.
Changes looks good. I'll merge it.