Fixed: Use indexer errors from response if Content-Type is XML before processing response

(cherry picked from commit 9bdc6183663a3510e53433a30ad701065e7ee9d9)

src/NzbDrone.Core/Indexers/Definitions/Headphones/HeadphonesRssParser.cs

@@ -54,7 +54,8 @@ namespace NzbDrone.Core.Indexers.Headphones
 
         protected override bool PreProcess(IndexerResponse indexerResponse)
         {
-            if (indexerResponse.HttpResponse.HasHttpError)
+            if (indexerResponse.HttpResponse.HasHttpError &&
+                (indexerResponse.HttpResponse.Headers.ContentType == null || !indexerResponse.HttpResponse.Headers.ContentType.Contains("xml")))
             {
                 base.PreProcess(indexerResponse);
             }

src/NzbDrone.Core/Indexers/Definitions/Newznab/NewznabRssParser.cs

@@ -58,7 +58,8 @@ namespace NzbDrone.Core.Indexers.Newznab
 
         protected override bool PreProcess(IndexerResponse indexerResponse)
         {
-            if (indexerResponse.HttpResponse.HasHttpError)
+            if (indexerResponse.HttpResponse.HasHttpError &&
+                (indexerResponse.HttpResponse.Headers.ContentType == null || !indexerResponse.HttpResponse.Headers.ContentType.Contains("xml")))
             {
                 base.PreProcess(indexerResponse);
             }

src/NzbDrone.Core/Indexers/Definitions/Torznab/TorznabRssParser.cs

@@ -28,7 +28,8 @@ namespace NzbDrone.Core.Indexers.Torznab
 
         protected override bool PreProcess(IndexerResponse indexerResponse)
         {
-            if (indexerResponse.HttpResponse.HasHttpError)
+            if (indexerResponse.HttpResponse.HasHttpError &&
+                (indexerResponse.HttpResponse.Headers.ContentType == null || !indexerResponse.HttpResponse.Headers.ContentType.Contains("xml")))
             {
                 base.PreProcess(indexerResponse);
             }