Xref-queries always 404

[RickardSjogren]

It's not possible to perform cross-reference queries (version 1.0.3, Python 3.6.1). For instance the query:
https://pubchem.ncbi.nlm.nih.gov/rest/pug/substance/xref/PatentID/US6187568B1/sids/JSON

Can be formulated as:

import pubchempy as pcp
response = pcp.request('US6187568B1', 'PatentID', 'substance', operation='sids', searchtype='xref')

However, this request 404:s since the ID is sent as payload data instead of inserted in the URL here.

[RickardSjogren]

A fix would be to change these lines to:

if namespace in ['listkey', 'formula', 'sourceid'] or searchtype == 'xref' or (searchtype and namespace == 'cid') or domain == 'sources':
    urlid = quote(identifier.encode('utf8'))

Then it would be nice to be able to use the short-hand:
sids = pcp.get_sids('US6187568B1', 'PatentID', 'substance', searchtype='xref'),
But doing this produces the wrong query since the operation is explicitly set None in get when setting searchtype.
However, passing on searchtype breaks TestRequest.test_listkey_requests