remove c# 7.1 feature in order to build

Author: karlovnv

src/progaudi.tarantool/RequestWriter.cs

@@ -131,7 +131,7 @@ bool GetRequest(out ArraySegment<byte> result)
                     }
                 }
 
-                result = default;
+                result = default(ArraySegment<byte>);
                 return false;
              }