fmt: fix inadvertent change to %#v

The reordering speedup in CL 6245068 changed the semantics
of %#v by delaying the clearing of some flags.  Restore the old
semantics and add a test.

Fixes #3706.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6302048

src/pkg/fmt/fmt_test.go

@@ -375,6 +375,7 @@ var fmttests = []struct {
 	{"%#v", &iarray, `&[4]interface {}{1, "hello", 2.5, interface {}(nil)}`},
 	{"%#v", map[int]byte(nil), `map[int]uint8(nil)`},
 	{"%#v", map[int]byte{}, `map[int]uint8{}`},
+	{"%#v", "foo", `"foo"`},
 
 	// slices with other formats
 	{"%#x", []int{1, 2, 15}, `[0x1 0x2 0xf]`},

src/pkg/fmt/print.go

@@ -734,6 +734,19 @@ func (p *pp) printField(field interface{}, verb rune, plus, goSyntax bool, depth
 		return false
 	}
 
+	// Clear flags for base formatters.
+	// handleMethods needs them, so we must restore them later.
+	// We could call handleMethods here and avoid this work, but
+	// handleMethods is expensive enough to be worth delaying.
+	oldPlus := p.fmt.plus
+	oldSharp := p.fmt.sharp
+	if plus {
+		p.fmt.plus = false
+	}
+	if goSyntax {
+		p.fmt.sharp = false
+	}
+
 	// Some types can be done without reflection.
 	switch f := field.(type) {
 	case bool:
@@ -775,6 +788,9 @@ func (p *pp) printField(field interface{}, verb rune, plus, goSyntax bool, depth
 		p.fmtBytes(f, verb, goSyntax, depth)
 		wasString = verb == 's'
 	default:
+		// Restore flags in case handleMethods finds a Formatter.
+		p.fmt.plus = oldPlus
+		p.fmt.sharp = oldSharp
 		// If the type is not simple, it might have methods.
 		if wasString, handled := p.handleMethods(verb, plus, goSyntax, depth); handled {
 			return wasString