add new parameter settings to sendAJAX function

Some sites don't accept the default content-type "application/x-www-form-urlencoded", must send out the specific content-type.
so we need the extra settings to override the default content-type setting in current sendAJAX implementation.

For back-compatible, I didn't modify the previous parameters, just add a new Object settings  as the last parameter. We can use it for storing more settings in the future if needed.