mDNS library does not send IPv6 packets and does not provide AAAA records

### Basic Infos

- [X] This issue complies with the [issue POLICY doc](https://github.com/esp8266/Arduino/blob/master/POLICY.md).
- [X] I have read the documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest) and the issue is not addressed there.
- [X] I have tested that the issue is present in current master branch (aka latest git).
- [X] I have searched the issue tracker for a similar issue.
- [N/A] If there is a stack dump, I have decoded it.
- [X] I have filled out all fields below.

#### Platform

- Hardware: ESP12F Breakout (Adafruit)
- Core Version: 2019-05-23
- Development Env: [Arduino IDE]
- Operating System: [MacOS]

### Settings in IDE

- Module: [Generic ESP8266 Module]
- Flash Mode: [qio]
- Flash Size: [4MB]
- lwip Variant: [v2 IPv6 Higher Bandwidth]
- Reset Method: [nodemcu]
- Flash Frequency: [40Mhz]
- CPU Frequency: [160MHz]
- Upload Using: [SERIAL]
- Upload Speed: [460800] (serial upload only)

### Problem Description

Running mDNS on an ESP8266 with an IPv6 enabled sketch only transmits IPv4 mDNS packets and even those packets do not include AAAA information for the IPv6 address.

### [MCVE](https://stackoverflow.com/help/mcve) Sketch
Reproduceable with example sketches.

### Debug Messages

```
N/A
```

Further information:
tcpdump -v -n -s 1500 ether host and port 5353
193.0.27.131.5353 > 224.0.0.251.5353: 0*- [0q] 4/0/1 _services._dns-sd._udp.local. PTR _arduino._tcp.local., _arduino._tcp.local. PTR gr8LED-701777._arduino._tcp.local., gr8LED-701777._arduino._tcp.local. (Cache flush) SRV gr8LED-701777.local.:8266 0 0, gr8LED-701777._arduino._tcp.local. (Cache flush) TXT "auth_upload=yes" "board="ESP8266_ESP12"" "ssh_upload=no" "tcp_check=no" (252)
08:27:49.924159 IP (tos 0x0, ttl 255, id 17, offset 0, flags [none], proto UDP (17), length 114)
193.0.27.131.5353 > 224.0.0.251.5353: 0*- [0q] 2/0/0 gr8LED-701777.local. (Cache flush) A 193.0.27.131, 131.27.0.193.in-addr.arpa. (Cache flush) PTR gr8LED-701777.local. (86)
08:27:49.924986 IP (tos 0x0, ttl 255, id 18, offset 0, flags [none], proto UDP (17), length 280)
193.0.27.131.5353 > 224.0.0.251.5353: 0*- [0q] 4/0/1 _services._dns-sd._udp.local. PTR _arduino._tcp.local., _arduino._tcp.local. PTR gr8LED-701777._arduino._tcp.local., gr8LED-701777._arduino._tcp.local. (Cache flush) SRV gr8LED-701777.local.:8266 0 0, gr8LED-701777._arduino._tcp.local. (Cache flush) TXT "auth_upload=yes" "board="ESP8266_ESP12"" "ssh_upload=no" "tcp_check=no" (252)
08:27:50.846542 IP (tos 0x0, ttl 255, id 19, offset 0, flags [none], proto UDP (17), length 114)
193.0.27.131.5353 > 224.0.0.251.5353: 0*- [0q] 2/0/0 gr8LED-701777.local. (Cache flush) A 193.0.27.131, 131.27.0.193.in-addr.arpa. (Cache flush) PTR gr8LED-701777.local. (86)
08:27:50.948694 IP (tos 0x0, ttl 255, id 20, offset 0, flags [none], proto UDP (17), length 280)
193.0.27.131.5353 > 224.0.0.251.5353: 0*- [0q] 4/0/1 _services._dns-sd._udp.local. PTR _arduino._tcp.local., _arduino._tcp.local. PTR gr8LED-701777._arduino._tcp.local., gr8LED-701777._arduino._tcp.local. (Cache flush) SRV gr8LED-701777.local.:8266 0 0, gr8LED-701777._arduino._tcp.local. (Cache flush) TXT "auth_upload=yes" "board="ESP8266_ESP12"" "ssh_upload=no" "tcp_check=no" (252)
08:27:51.871012 IP (tos 0x0, ttl 255, id 21, offset 0, flags [none], proto UDP (17), length 114)
193.0.27.131.5353 > 224.0.0.251.5353: 0*- [0q] 2/0/0 gr8LED-701777.local. (Cache flush) A 193.0.27.131, 131.27.0.193.in-addr.arpa. (Cache flush) PTR gr8LED-701777.local. (86)
08:27:51.975507 IP (tos 0x0, ttl 255, id 22, offset 0, flags [none], proto UDP (17), length 280)

Note: no IPv6 packets recorded.

Also:
odmbpro2:src owendelong$ ping esp8266-701777.local
PING esp8266-701777.local (193.0.27.131): 56 data bytes
64 bytes from 193.0.27.131: icmp_seq=0 ttl=255 time=41.600 ms
64 bytes from 193.0.27.131: icmp_seq=1 ttl=255 time=4.782 ms
64 bytes from 193.0.27.131: icmp_seq=2 ttl=255 time=5.351 ms
^C
--- esp8266-701777.local ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 4.782/17.244/41.600/17.224 ms
odmbpro2:src owendelong$ ping6 esp8266-701777.local
ping6: getaddrinfo -- nodename nor servname provided, or not known
odmbpro2:src owendelong$

And:
odmbpro2:src owendelong$ dns-sd -q gr8led-701777.local AAAA IN
DATE: ---Thu 23 May 2019---
8:27:24.490 ...STARTING...
^C
odmbpro2:src owendelong$ dns-sd -q gr8led-701777.local A IN
DATE: ---Thu 23 May 2019---
8:41:56.222 ...STARTING...
Timestamp A/R Flags if Name Type Class Rdata
8:41:56.223 Add 2 5 gr8LED-701777.local. Addr IN 193.0.27.131