Library throws exception using the default parameter for certificate

[elbosso]

When falling back to the vertificate inside the reply for verification like so:

import rfc3161ng
rt = rfc3161ng.RemoteTimestamper('http://time.certum.pl', include_tsa_certificate=True)
tst = rt.timestamp(data=b'John Doe')
rt.check(tst, data=b'John Doe')

an exception is thrown because the code can not handle parameter certificate in load_certificate being None:

Traceback (most recent call last):
  File "rfc.py", line 4, in <module>
    rt.check(tst, data=b'John Doe')
  File "venv/lib/python3.6/site-packages/rfc3161ng/api.py", line 229, in check
    hashname=self.hashname,
  File "venv/lib/python3.6/site-packages/rfc3161ng/api.py", line 142, in check_timestamp
    certificate = load_certificate(signed_data, certificate)
  File "venv/lib/python3.6/site-packages/rfc3161ng/api.py", line 123, in load_certificate
    if b'-----BEGIN CERTIFICATE-----' in certificate:
TypeError: argument of type 'NoneType' is not iterable

[trbs]

Could you please redo the commits and remove all other changes not relevant to the issue ?

Also it would be really helpful to have a small test case that triggers this error so we can be sure that we fixed the issue and that we do not regress later.

Thanks !