pandas extension pg_copy_from silently converts text objects to str

[ecsalomon]

As a user, I expect, because I am also passing a sqlalchemy connectable, that pg_copy_from would respect the properties of a text object passed as the query (including using the default values of any bound parameters at execution), but it silently converts the text object to a plain str.

A couple of solutions are possible:

1. Document that the copy extensions convert things to string (and maybe give a warning when getting something other than a string)
2. Support text objects in a way compatible with user expectations of sqlalchemy

2 seems nicer but 🤷