Refactor to recommended header usage

Author: commonsensesoftware

src/AspNetCore/WebApi/src/Asp.Versioning.Http/DefaultApiVersionReporter.cs

@@ -21,7 +21,7 @@ private static void AddApiVersionHeader( IHeaderDictionary headers, string heade
 
         if ( versions.Count == 1 )
         {
-            headers.Add( headerName, versions[0].ToString() );
+            headers[headerName] = versions[0].ToString();
             return;
         }
 
@@ -55,7 +55,7 @@ private static void AddApiVersionHeader( IHeaderDictionary headers, string heade
             }
         }
 
-        headers.Add( headerName, headerValue.ToString() );
+        headers[headerName] = headerValue.ToString();
         pool.Return( array );
     }
 }

src/AspNetCore/WebApi/src/Asp.Versioning.Http/Http/HttpResponseExtensions.cs

@@ -38,7 +38,7 @@ public static void WriteSunsetPolicy( this HttpResponse response, SunsetPolicy s
 
         if ( sunsetPolicy.Date.HasValue )
         {
-            headers.Add( Sunset, sunsetPolicy.Date.Value.ToString( "r" ) );
+            headers[Sunset] = sunsetPolicy.Date.Value.ToString( "r" );
         }
 
         AddLinkHeaders( headers, sunsetPolicy.Links );
@@ -53,16 +53,7 @@ private static void AddLinkHeaders( IHeaderDictionary headers, IList<LinkHeaderV
             values[i] = links[i].ToString();
         }
 
-        var newValues = new StringValues( values );
-
-        if ( headers.TryGetValue( Link, out var existingValues ) )
-        {
-            headers[Link] = StringValues.Concat( existingValues, newValues );
-        }
-        else
-        {
-            headers.Add( Link, newValues );
-        }
+        headers.Append( Link, values );
     }
 
     /// <summary>