Don't try to optimize method and version string allocations on big endian.

Author: Cesar Blum Silveira

src/Microsoft.AspNet.Server.Kestrel/Infrastructure/MemoryPoolIterator2Extensions.cs

@@ -280,6 +280,13 @@ public static ArraySegment<byte> GetArraySegment(this MemoryPoolIterator2 start,
         public static bool GetKnownString(this MemoryPoolIterator2 begin, MemoryPoolIterator2 end, out string knownString)
         {
             knownString = null;
+
+            // This optimization only works on little endian environments (for now).
+            if (!BitConverter.IsLittleEndian)
+            {
+                return false;
+            }
+
             var inputLength = begin.GetLength(end);
 
             if (inputLength > sizeof(long))