Adding a faster check for GetUTF8ByteCount for netcoreapp3.1

Author: Stefán J. Sigurðarson

projects/RabbitMQ.Client/client/impl/WireFormatting.cs

@@ -498,7 +498,11 @@ public static int GetFieldValueByteCount(object value)
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
+#if NETCOREAPP
+        public static int GetUTF8ByteCount(ReadOnlySpan<char> val) => val.IsEmpty ? 0 : Encoding.UTF8.GetByteCount(val);
+#else
         public static int GetUTF8ByteCount(string val) => string.IsNullOrEmpty(val) ? 0 : Encoding.UTF8.GetByteCount(val);
+#endif
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static int WriteLong(Span<byte> span, uint val)