builder: handle cases where onbuild is not uppercase.

closes #8675 

/cc @tiborvass 
This is a pretty long multiline string when it fails; is this really necessary?

> On Oct 21, 2014, at 12:41 PM, Tibor Vass <notifications@github.com> wrote:
> 
> In integration-cli/docker_cli_build_test.go:
> 
> > +  `, true)
> > +
> > +	if err != nil {
> > +		t.Fatal(err)
> > +	}
> > +
> > +	_, out, err := buildImageWithOut(name2, fmt.Sprintf(`
> > +  FROM %s
> > +  `, name), true)
> > +
> > +	if err != nil {
> > +		t.Fatal(err)
> > +	}
> > +
> > +	if !strings.Contains(out, "quux") {
> > +		t.Fatalf("Did not receive the expected echo text, got %s", out)
> %q
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/docker/docker/pull/8684/files#r19173632>.
>