feat: Test object returned by dbUnquoteIdentifier() is of class Id

[MichaelChirico]

Closes #403

I haven't dived in to how unit testing is done for {DBItest}, any advice would be helpful. I searched for "quote" in the testthat/ folder with no hits, so I didn't see an easy way to shoehorn my way in.

I did notice this floating comment, it might be referring to the code in question?

DBItest/R/spec-sql-unquote-identifier.R

Lines 57 to 58 in f19fb4d

	#' (For backends it may be most convenient to return [Id] objects
	#' to achieve this behavior, but this is not required.)

But if so, maybe it's outdated? Since as noted, dbUnquoteIdentifier() is documented to return Id.

This was added to {DBItest} (Mar. 2018) not long after [Id] was documented as the return for dbUnquoteIdentifier() (Feb. 2018):

r-dbi/DBI@8f2a2e9

e291f73

[krlmlr]

Thanks!