When a checkbox had no /AP key (which is not mandatory)
poppler was silently ignoring the setState() call that
changes the checked/unchecked state.

Fixed by using getOnStr() instead of accessing onStr
directly, as the former has code in place to return
correct values when the field is a checkbox and has
no names for the On/Off states (as a result of not
having the /AP key which could contain those names).

A testcase is included. An example definition of an
affected checkbox follows:

/F 4
/FT /Btn
/H /P
/MK /BC [1,0,0] /BG [1,1,1] /CA (4)
/Q 0
/Rect [235.277,654.247,249.224,668.194]
/Subtype /Widget
/T (basiccheckbox)
/Type /Annot
/V /Off

Fixes issue #655