support nil bulk string

ewwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww

values.go

@@ -101,6 +101,9 @@ func readBulkString(prefix []byte, r io.Reader) (value, error) {
 		return nil, fmt.Errorf("unable to read bulkstring in redis protocol: bad prefix: %v", err)
 	}
 
+	if n == -1 {
+		return nil, nil
+	}
 	n += 2
 	b := make([]byte, n)
 	n_read, err := r.Read(b)

values_test.go

@@ -47,6 +47,7 @@ var valueTests = []valueTest{
 	{"-one two ", Error("one two ")}, // trailing space
 	{"- one two", Error(" one two")}, // leading space
 
+	{"$-1\r\n", nil},
 	{"$0\r\n\r\n", BulkString("")}, // is this even a thing?
 	{"$1\r\nx\r\n", BulkString("x")},
 	{"$4\r\netsy\r\n", BulkString("etsy")},