Maps rely on the keys being comparable.
Using an interface type as the map key is dangerous,
because some interface types are not comparable.
I talked about this in my "Stupid Gopher Tricks" talk:
https://talks.golang.org/2015/tricks.slide
In this case, if the user-provided writer is backed by a slice
(such as io.MultiWriter) then the code will panic at run time.
Signed-off-by: Andrew Gerrand <adg@golang.org>
