Fix error when downloading large file

The Livescribe SmartPen firmware (at least v1.5) requires a Connection header after every Continue response (which contradicts the Obex spec). We convince openobex to send the header by calling OBEX_ObjectAddHeader in the event callback.

The problem is that openobex keeps counting request bytes, and when you set OBEX_FL_FIT_ONE_PACKET it will return error once all your Connection headers reach the 1kB MTU. The solution is to turn off that flag.

Example of getting error when the pen has a lot of data (in my case, PaperReplay is 1GB):

    >>> import pysmartpen
    >>> s = pysmartpen.Smartpen()
    >>> s.connect(vendor=0x1cfb, product=0x1010)
    >>> s.get_paperreplay('paperreplay')
    oah fail -1
    oah fail -1
    FAIL 53 3
    python: smartpen.c:83: obex_event: Assertion `0' failed.
    Aborted (core dumped)