encoding/json: add test for Unmarshal of malformed data

Roll back CL making primitive type unmarshal faster,
because it broke the Unmarshal of malformed data.

Add benchmarks for unmarshal of primitive types.

Update #3949.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7228061

src/pkg/encoding/json/decode.go

@@ -52,25 +52,6 @@ import (
 // an UnmarshalTypeError describing the earliest such error.
 //
 func Unmarshal(data []byte, v interface{}) error {
-
-	// skip heavy processing for primitive values
-	var first byte
-	var i int
-	for i, first = range data {
-		if !isSpace(rune(first)) {
-			break
-		}
-	}
-	if first != '{' && first != '[' {
-		rv := reflect.ValueOf(v)
-		if rv.Kind() != reflect.Ptr || rv.IsNil() {
-			return &InvalidUnmarshalError{reflect.TypeOf(v)}
-		}
-		var d decodeState
-		d.literalStore(data[i:], rv.Elem(), false)
-		return d.savedError
-	}
-
 	d := new(decodeState).init(data)
 
 	// Quick check for well-formedness.