When I step through cmislib's checkIn unit test against a preview build of 4.2 Enterprise using the new service URLs, I see that the PUT that does the checkin is hitting this URL:
u'http://localhost:8080/alfresco/api/-default-/public/cmis/versions/1.0/atom/entry?id=71be4c86-1dcb-49a8-8373-d1c5ea4405b7%3Bpwc'
So try changing "content" to "entry" and also double-check that you have the ID of the PWC. Note that mine actually has "pwc" on the end whereas yours does not.