Fix duplicate http client tracing headers

[trask]

Adds a new http client test, and fixes a few http client instrumentations that did not pass:

  // this test verifies two things:
  // * the javaagent doesn't cause multiples of tracing headers to be added
  //   (TestHttpServer throws exception if there are multiples)
  // * the javaagent overwrites the existing tracing headers
  //   (so that it propagates the same trace id / span id that it reports to the backend
  //   and the trace is not broken)
  def "request with existing tracing headers"()