Correctly handle string table names with fallback langs

Prior to this change, if you passed a custom table name and used a language where a fallback language was used to determine the pluralization rule, the tableName variable would be passed to `[self pathForResource:...]` multiple times. On the second time, passing an absolute string would produce a nil result.

This would happen, for example, for Mexican Spanish, where the lang list is ("es_MX", "es-MX", "es", "English"), and "es" determines the pluralization rule.