fix datarace for parallel http client request

[t-yuki]

There is a datarace problem on xray/segement_model.go when I use xray RoundTripper for parallel requests in a http handler like:

client -IncomingHeader-> handler
                            |-RoundTrip-> request1
                            |-RoundTrip-> request2

Since Segment.IncomingHeader object is shared across requests, modifying IncomingHeader in DownstreamHeader func will cause a datarace.
In this patch, DownstreamHeader never rewrite original IncomingHeader object.
Instead, it returns a modified copy of IncomingHeader for each call if available.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.