Remove crufty slicing of subvalues

[mjordan]

Looks like I introduced unnecessary slicing of subvalues in logic that deals with cardinality of field values. Specifically, if the cardinality of a field is -1 (unlimited), I'm slicing on this value, which results in removing the last item in the list.

I will take care of this once issue-20 branch is merged.

[mjordan]

Plus, where we do slice in the update task, we need to slice so that we include any existing field values.

[mjordan]

Part of this issue should be adding a test that field values are added/updated as expected.

[mjordan]

Initial issue fixed, going to create a new one from this.