Should use Set() instead of Add() in HTTPHeadersCarrier

[jeremyxu2010]

Should use Set() instead of Add() in HTTPHeadersCarrier.

Resolves #159

[yurishkuro]

I agree that this is probably the right change, but it might break users who already depend on the existing behavior (e.g. by writing the same header multiple times to the carrier).

@tedsuo

[szuecs]

@yurishkuro It breaks the contract of the wireformat, so this is IMO a bug fix or am I wrong?

On the other hand the JVM lightstep library parse the data and crashes if there is a list of string in the header. I am interested, how other libraries will handle this case.
We just had a prodcution incident in our CI/CD UI, because of that.
JVM lib from lightstep says:

 WARN [manifold-pool-2-755] c-l-s.tracing - Unable to parse parent span information {"ot-tracer-spanid" "<spanid1>,<spanid2>", "ot-tracer-traceid" "<traceid1>,<traceid2>", "ot-tracer-sampled" "true,true"}.
                                     java.lang.Thread.run                 Thread.java: 748
                                                      ...                                 
                 manifold.executor/thread-factory/reify/f                executor.clj:  44
                 io.aleph.dirigiste.Executor$Worker$1.run               Executor.java:  62
                                                      ...                                 
            aleph.http.server/handle-request/fn/f--auto--                  server.clj: 157
      cdp-log-service.tracing/wrap-parent-span-context/fn                 tracing.clj: 131
            cdp-log-service.tracing/headers->span-context                 tracing.clj: 127
                                                      ...                                 
       com.lightstep.tracer.shared.AbstractTracer.extract         AbstractTracer.java: 362
com.lightstep.tracer.shared.HttpHeadersPropagator.extract  HttpHeadersPropagator.java:   9
com.lightstep.tracer.shared.HttpHeadersPropagator.extract  HttpHeadersPropagator.java:  15
    com.lightstep.tracer.shared.TextMapPropagator.extract      TextMapPropagator.java:  10
    com.lightstep.tracer.shared.TextMapPropagator.extract      TextMapPropagator.java:  42
           com.lightstep.tracer.shared.Util.fromHexString                   Util.java:  57
                              java.math.BigInteger.<init>             BigInteger.java: 479
                               java.lang.Integer.parseInt                Integer.java: 580
           java.lang.NumberFormatException.forInputString  NumberFormatException.java:  65
java.lang.NumberFormatException: For input string: "49de,46"

A colleague wrote
lightstep/lightstep-tracer-go#160, for lightstep, but this seems more a workaround for one product, if you agree that this is a bug.