Fix resolving of nested Data containing JSON

[svenmuennich]

Before the performance refactoring added via #2 it was possible to decode a JSON value that contained nested Data values, which themselves contained valid JSON:

let json = JSON([
    "root": #"{"foo": "bar"}"#.data(using: .utf8)!
])

This is an edge case, since it can only happen if the JSON content was constructed in code or at runtime (not when decoding e.g. from a String), but we use this feature ourselves.

#2 broke that feature. This PR fixes it.

[felixbrucker]

LGTM