fix(ConfigurableMessageHandler): Only validate headers on first attempt.

Author: robertvoinescu-work

Src/Support/Google.Apis.Auth/OAuth2/AccessTokenWithHeaders.cs

@@ -17,6 +17,7 @@ limitations under the License.
 using Google.Apis.Util;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Linq;
 using System.Net.Http;
 using System.Net.Http.Headers;
 
@@ -78,12 +79,14 @@ public void AddHeaders(HttpRequestHeaders requestHeaders)
 
             foreach (var header in Headers)
             {
+                // Overwite any pre-existing header with the provided values.
+                requestHeaders.Remove(header.Key);
                 requestHeaders.Add(header.Key, header.Value);
             }
         }
 
         /// <summary>
-        ///  Adds the headers in this object to the given request.
+        ///  Adds the headers in this object to the given request. Overwriting any already present header fields.
         /// </summary>
         /// <param name="request">The request to add the headers to.</param>
         public void AddHeaders(HttpRequestMessage request) => 

Src/Support/Google.Apis.Core/Http/ConfigurableMessageHandler.cs

@@ -466,7 +466,12 @@ protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage
 
                 // Before having the credential intercept the call, check that quota project hasn't
                 // been added as a header. Quota project cannot be added except through the credential.
-                CheckValidAfterInterceptors(request);
+                // We only perform this check on the first attempt; on retries the header may be 
+                // present from a previous attempt.
+                if (triesRemaining == maxRetries) // This is the first attempt
+                {
+                    CheckValidAfterInterceptors(request);
+                }
 
                 await CredentialInterceptAsync(request, cancellationToken).ConfigureAwait(false);