Use discoverSchema for the case where options.schema is not present

/to @ritch 

When options.schema is not passed, the discovery defaults to a given schema. The key of the returned definitions will be schema.table. Using options.schema +'.'+ table will not work.
> It is a backwards-compatible change.

@raymondfeng can you please explain little bit more why `result[options.schema + '.' + name]`  is the same value as `result`, considering the case where both `options.schema` and `name` have a valid value?