Add support for ldp:MemberSubject

[acoburn]

Environment

Operating system: OS X

Python version: 3.6

LAKEsuperior release, branch, or commit #: alpha 14

Steps to reproduce

Create an Indirect Container with the triple:

<> ldp:insertedContentRelation ldp:MemberSubject .

Now add child resources to the indirect container via PUT or POST.

Observed behavior

500 Error Response

Expected behavior

200 Response. But more importantly, the membership resource should then be populated with member triples exactly as if the indirect container were a direct container.

Other notes worth mentioning

You may want to read up on the use of ldp:MemberSubject in the LDP spec.