Some string values such as "_nil_" have special meaning

[niemeyer]

As discovered via #1477, some strings may have special meaning to the server, which is usually a very bad idea as it mixes user data with implementation logic in unexpected ways. A string such as _nil_ for example might very easily be part of the data imported into the system, and it would magically disappear once stored.

As a recommendation, I would make SetValueString in the Go code respect the input value exactly. Even escaping should be applied internally, instead of expecting developers to escape the value by themselves. If the low-level functionality is necessary for certain tools, I would make that explicit in the API name, such as SetValueRaw or similar, and make the rules there more clear so people can tell what to expect when using it.

[pawanrawal]

Thats a good point @niemeyer. I will do a proper fix for this soon.

[peterstace]

I've made some changed in a branch fix_nil_string that I created while working on #1627 . They might be useful when we work on this. It will probably be easier to work on this issue once the client/transaction work has been finished -- since handling empty strings will be easier when we no longer delete an edge by not setting the object (i.e leave it as an empty string).

[pawanrawal]

A fix was merging with 56c99ea.