Fix #737 - Loosen GetDbProviderManifestToken behavior

Convert the cast to an 'as'. Seems reasonable to not cause the
application fail due to a bad interaction in what seems to be a lifetime
issue. Could also log a warning should you want to track this unexpected
case.
Merge pull request #836 from jasonkenneth/Fix737

Fix #737 - Loosen GetDbProviderManifestToken behavior