MDNS text record encoding is not RFC compliant

[matthewgream]

To reproduce:

• review ArduinoMDNS code in sendMDNSmessage
• observe the text record is encoded with only 16 bit length and then raw string
• read https://www.ietf.org/rfc/rfc6763.txt to understand that text record encoding is specifically different in MDNS packets
• read that text records must be inserted with each string prefixed by single byte length of string
• observe that this is not carried out in ArduinoMDNS