relax naming rule for index key name

[rieschl]

Q	A
Type	improvement
BC Break	no

Summary

As discussed in Slack I propose to loosen the index key name from NMTOKEN to string.

TL;DR: Sometimes field names have characters that aren't covered by NMTOKEN and setting an index on such a field becomes impossible. MongoDB has no problems with field names or indexes containing arbitrary characters (e.g. a \).

I have no idea how to write a test for that, though.

[alcaeus]

Thanks @rieschl!