Prevent X-Amzn-Trace-Id from Being Added More Than Once (#40)

* Prevent X-Amzn-TraceId from Being Added More Than Once

Under some circumstances, the chain of calls through
`DelegatingHandler` instances can reach a particular handler more than
once. When that handler is `HttpClientXRayTracingHandler`, it produces
an X-Amzn-Trace-Id header that cannot be parsed because it is
_multiple_ headers. (As represented in a string by separating them
with ", ".)

This patch checks whether X-Amzn-Trace-Id has already been added, and
skips adding another if it already has been.

* Replaces Existing X-Amzn-Trace-Id Header, If Present

sdk/src/Handlers/System.Net/Utils/RequestUtil.cs

@@ -24,7 +24,13 @@ internal static void ProcessRequest(WebRequest request)
         /// <param name="request">An instance of <see cref="HttpRequestMessage"/></param>
         internal static void ProcessRequest(HttpRequestMessage request)
         {
-            ProcessRequest(request.RequestUri, request.Method.Method, header => request.Headers.Add(TraceHeader.HeaderKey, header));
+            ProcessRequest(request.RequestUri, request.Method.Method, AddOrReplaceHeader);
+
+            void AddOrReplaceHeader(string header)
+            {
+                request.Headers.Remove(TraceHeader.HeaderKey);
+                request.Headers.Add(TraceHeader.HeaderKey, header);
+            }
         }
 
         /// <summary>

sdk/test/UnitTests/HttpClientXRayTracingHandlerTests.cs

@@ -55,7 +55,7 @@ public async Task TestSendAsync()
             var segment = TraceContext.GetEntity();
             AWSXRayRecorder.Instance.EndSegment();
 
-            var traceHeader = request.Headers.GetValues(TraceHeader.HeaderKey).FirstOrDefault();
+            var traceHeader = request.Headers.GetValues(TraceHeader.HeaderKey).SingleOrDefault();
             Assert.IsNotNull(traceHeader);
 
             var requestInfo = segment.Subsegments[0].Http["request"] as Dictionary<string, object>;
@@ -77,7 +77,7 @@ public async Task Test404SendAsync()
             var segment = TraceContext.GetEntity();
             AWSXRayRecorder.Instance.EndSegment();
 
-            var traceHeader = request.Headers.GetValues(TraceHeader.HeaderKey).FirstOrDefault();
+            var traceHeader = request.Headers.GetValues(TraceHeader.HeaderKey).SingleOrDefault();
             Assert.IsNotNull(traceHeader);
 
             var requestInfo = segment.Subsegments[0].Http["request"] as Dictionary<string, object>;