This is a slight change to fmt's semantics, but means that if you use
%d to print an integer with a Stringable value, it will print as an integer.
This came up because Time.Month() couldn't cleanly print as an integer
rather than a name. Using %d on Stringables is silly anyway, so there
should be no effect outside the fmt tests.
As a mild bonus, certain recursive failures of String methods
will also be avoided this way.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5453053