Handle default Predictor value properly

An earlier commit set the default Predictor value to 1, as per PDF
32000-1:2008 - but failed to actually handle this value.  It's equivalent
to PNG None (a value of 10), so should be handled as such.  This fixes
issue #5
Fix incorrect return of xref entry if 0 and object not found

If the xref table lists an object as having an offset of 0 and the strpos
check returns false (i.e. the object is in an objstream or simply not
there), findObjectOffset was incorrectly returning 0 instead of false.
Changing the check to === to ensure that both values are of the same type
avoids this issue and allows the calling function to handle this case
properly.  This was discovered in the sample PDF provided with issue #5