Edited .update to accept updates if node._id is 0

The current if-clause for checking the existence of node property _id does not take into account that integer values of 0 is equivalent to the boolean value of false.
This means that a supplied _id of 0 will be seen as a falsy id.
The proposed fix handles this case.
Merge pull request #5 from MickeJagekrans/patch-1

Edited write.update to accept updates if node._id == 0