Fix namecheap MX record support.

[doddo]

Creating MX records with namecheap used to crash like this:

lexicon  namecheap create peppe.club MX  --name "@" --content "10 smtp.peppe.club"
Remote: 8
To set: 9
Traceback (most recent call last):
  File "/home/petter/git/lexicon/venv/bin/lexicon", line 11, in <module>
    load_entry_point('dns-lexicon==3.3.22', 'console_scripts', 'lexicon')()
  File "/home/petter/git/lexicon/venv/lib/python3.8/site-packages/dns_lexicon-3.3.22-py3.8.egg/lexicon/cli.py", line 117, in main
    results = client.execute()
  File "/home/petter/git/lexicon/venv/lib/python3.8/site-packages/dns_lexicon-3.3.22-py3.8.egg/lexicon/client.py", line 84, in execute
    return self.provider.create_record(record_type, name, content)
  File "/home/petter/git/lexicon/venv/lib/python3.8/site-packages/dns_lexicon-3.3.22-py3.8.egg/lexicon/providers/base.py", line 80, in create_record
    return self._create_record(rtype, name, content)
  File "/home/petter/git/lexicon/venv/lib/python3.8/site-packages/dns_lexicon-3.3.22-py3.8.egg/lexicon/providers/namecheap.py", line 202, in _create_record
    self.client.domains_dns_addHost(self.domain, record)
  File "/home/petter/git/lexicon/venv/lib/python3.8/site-packages/namecheap.py", line 388, in domains_dns_addHost
    self._call("namecheap.domains.dns.setHosts", extra_payload)
  File "/home/petter/git/lexicon/venv/lib/python3.8/site-packages/namecheap.py", line 144, in _call
    xml = self._fetch_xml(payload, extra_payload)
  File "/home/petter/git/lexicon/venv/lib/python3.8/site-packages/namecheap.py", line 137, in _fetch_xml
    raise ApiError(error.attrib['Number'], error.text)
namecheap.ApiError: 2050900 - INVALID_ADDR : ' 10 smtp.peppe.club' should not be an IP/ URL for MX record.(host name: @)

Furthermore, listing them did not usually produce the priority field.

With this fix, it works:

% PYTHONPATH=. lexicon  namecheap create peppe.club MX  --name "@" --content "10 smtp.peppe.club"
Remote: 8
To set: 9
RESULT
------
True
% PYTHONPATH=. lexicon  namecheap list peppe.club MX
ID     TYPE NAME            CONTENT              TTL 
------ ---- --------------- -------------------- ----
746267 MX   @.peppe.club    10 smtp.peppe.club.  3600

[adferrand]

LGTM! Thanks a lot.