Allow non-string dict key types

[QuadratClown]

Javascript does not allow keys for objects to be other than string or symbol. However, node-dbus-next uses native js objects for the dbus dict format, which allow keys of any type. As keys in js objects are auto-converted to string, marshalling will throw an error for non-string dict key types.

This pull request simply adds a conversion in marshall-compat.js that auto-converts faulty strings keys back to their respective type.

This pull requests fixes https://github.com/dbusjs/node-dbus-next/issues/79