Fixed presence detection bug in utils.set_defaults

Previously, Utils.set_defaults was using `if(conf[keys[i]])`
to check for the presence of a configuration key.  This would
fail if `conf[keys[i]]` happened to be false.  Instead, we now
use `if(keys[i] in conf)`, which simply checks for the presence
of the key in the conf object.
Fixed Cursor URI Support Detection

There was a bug in cursor URI support detection due to the way
set_defaults now works -- the code was checking for `null`, whereas
when not set, options default to `undefined` unless otherwise
specified.  The code now checks for either `null` or `undefined`.
Tests have been added to ensure that this works properly.