Don't use assignment for MERGE/Update

[jexp]

See the discussion here:

#99 (review)

Switch from SET x = {...} to SET x += {} or individual set operations.
We could also consider putting the data just into a parameter instead. So it would become SET x += $data

Usually you merge on a single field (in exceptional cases on a set of fields). (they keys that are used to look-up nodes for uniqueness)
And update fields are the fields that are added/overriden.

As MERGE is get-or-create you don't want to destroy existing additional data that might have been added in between by other means.

And if you leave off fields it doesn't mean you want to remove them, just that you don't have data (or updates) for them at the moment.

[rbramley]

This causes a critical problem for data that has been loaded from CSV files where the existing external :ID field has been retained.
The result of an update mutation is that the ID field is stripped from the node.

[Andy2003]

@rbramley your issue was resolved by #95