Lines should be escaped before printing

[bbrks]

The following test produces an incorrect message when failing, as the lines passed to decorate() are not being escaped before being passed into log()'s Sprintf().

func TestFormatStringEscape(t *testing.T) {
	is := New(t)
	is.Equal("20% VAT", "0.2 VAT")
}

=== RUN   TestFormatStringEscape
	is_test.go:296: 20%!V(MISSING)AT != 0.2 VAT
--- FAIL: TestFormatStringEscape (0.00s)