BUG/MINOR: lua: Socket.send threw runtime error: 'close' needs 1 arguments.

BUG/MINOR: lua: Socket.send threw runtime error: 'close' needs 1 arguments.

Function `hlua_socket_close` expected exactly one argument on the Lua stack.
But when `hlua_socket_close` was called from `hlua_socket_write_yield`,
Lua stack had 3 arguments. So `hlua_socket_close` threw the exception with
message "'close' needs 1 arguments".

Introduced new helper function `hlua_socket_close_helper`, which removed the
Lua stack argument count check and only checked if the first argument was
a socket.

This fix should be backported to 1.8, 1.7 and 1.6.