Apply AppendEnfOfLine perf suggestion

Author: sebastienros

src/Servers/Kestrel/Core/src/Internal/Http/HttpParser.cs

@@ -308,16 +308,16 @@ public bool ParseHeaders(TRequestHandler handler, ref SequenceReader<byte> reade
 
     private static byte[] AppendEndOfLine(ReadOnlySpan<byte> span, bool lineFeedOnly)
     {
+        Debug.Assert(!span.IsEmpty);
+
         var array = new byte[span.Length + (lineFeedOnly ? 1 : 2)];
 
-        span.CopyTo(array);
-        array[^1] = ByteLF;
+        Debug.Assert(array.Length >= 2);
 
-        if (!lineFeedOnly)
-        {
-            array[^2] = ByteCR;
-        }
+        array[^2] = ByteCR;
+        array[^1] = ByteLF;
 
+        span.CopyTo(array);
         return array;
     }