When trying to output the `endpoint` attribute of an `aws_db_instance`,
the port was not being emitted properly. Converting the port number from
an integer to a string has resolved the issue.
cc/ @mitchellh
This one caught me a bit, and I'm not sure if there's another AttrType
that could be useful here – perhaps a AttrTypeHidden or something? I
don't know...this feels sloppy to have in the diff func.
Hector Castro