Change how we enable DNS over TLS option

We are removing the TLS atributte from Client type. Now if you want to enable
DNS over TLS you should use the value "tcp-tls", "tcp4-tls" or "tcp6-tls" in
Net attribute.

See #297

client.go

@@ -25,10 +25,9 @@ type Conn struct {
 
 // A Client defines parameters for a DNS client.
 type Client struct {
-	Net            string            // if "tcp" a TCP query will be initiated, otherwise an UDP one (default is "" for UDP)
+	Net            string            // if "tcp" or "tcp-tls" (DNS over TLS) a TCP query will be initiated, otherwise an UDP one (default is "" for UDP)
 	UDPSize        uint16            // minimum receive buffer for UDP messages
-	TLS            bool              // enables TLS connection (port 853)
-	TLSConfig      *tls.Config       // TLS connection configuration (TLS flag should be enabled)
+	TLSConfig      *tls.Config       // TLS connection configuration
 	DialTimeout    time.Duration     // net.DialTimeout, defaults to 2 seconds
 	ReadTimeout    time.Duration     // net.Conn.SetReadTimeout value for connections, defaults to 2 seconds
 	WriteTimeout   time.Duration     // net.Conn.SetWriteTimeout value for connections, defaults to 2 seconds
@@ -155,15 +154,27 @@ func (c *Client) writeTimeout() time.Duration {
 
 func (c *Client) exchange(m *Msg, a string) (r *Msg, rtt time.Duration, err error) {
 	var co *Conn
-
 	network := "udp"
-	if c.Net != "" {
-		network = c.Net
+	tls := false
+
+	switch c.Net {
+	case "tcp-tls":
+		network = "tcp"
+		tls = true
+	case "tcp4-tls":
+		network = "tcp4"
+		tls = true
+	case "tcp6-tls":
+		network = "tcp6"
+		tls = true
+	default:
+		if c.Net != "" {
+			network = c.Net
+		}
 	}
 
-	if c.TLS {
-		// TLS connection is always TCP
-		co, err = DialTimeoutWithTLS("tcp", a, c.TLSConfig, c.dialTimeout())
+	if tls {
+		co, err = DialTimeoutWithTLS(network, a, c.TLSConfig, c.dialTimeout())
 	} else {
 		co, err = DialTimeout(network, a, c.dialTimeout())
 	}