fmt: Fix signed zero-padding for positive floats

Space padding still has the same issue, I will send a separate patch for that
if this one gets accepted.
Fixes #6856.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/35660043

src/pkg/fmt/fmt_test.go

@@ -220,6 +220,8 @@ var fmtTests = []struct {
 	{"%+.3e", 0.0, "+0.000e+00"},
 	{"%+.3e", 1.0, "+1.000e+00"},
 	{"%+.3f", -1.0, "-1.000"},
+	{"%+07.2f", 1.0, "+001.00"},
+	{"%+07.2f", -1.0, "-001.00"},
 	{"% .3E", -1.0, "-1.000E+00"},
 	{"% .3e", 1.0, " 1.000e+00"},
 	{"%+.3g", 0.0, "+0"},

src/pkg/fmt/format.go

@@ -372,7 +372,10 @@ func (f *fmt) formatFloat(v float64, verb byte, prec, n int) {
 	default:
 		// There's no sign, but we might need one.
 		if f.plus {
-			slice[0] = '+'
+			f.buf.WriteByte('+')
+			f.wid--
+			f.pad(slice[1:])
+			return
 		} else if f.space {
 			// space is already there
 		} else {