Disambiguate conversion from JSON to string values.
The nlohmann::json library attempts to automatically determine
the type-conversion needed (from JSON to some other type) depending
on the context in which the JSON value is used. Because
smtk::string::Token
is now a valid conversion and has comparison
operations to strings, the context does not force a single type.
This causes the compiler to error out. Forcing the type required
from JSON fixes this issue.