Add conformance test for proxied and redirected binary content

[escowles]

Verify support for external content using message/external with URL and local-file access-types:

• message/external-body spec: https://www.w3.org/Protocols/rfc1341/7_3_Message.html
• access-type=url spec: https://tools.ietf.org/rfc/rfc2017.txt

[barmintor]

This part of the spec has changed a bit, but I spoke with @awoods about it today.

[barmintor]

@escowles is there a write-up of what the MODE alignment decision was re: proxied content?

[escowles]

@barmintor No, I don't think there's really been a firm agreement on that amongst the spec editors, and so I'm not exactly sure what the folks working on the MODE alignment sprints have done (or if they are waiting on us).

But my understanding of the current thinking is in these two tickets:

• Use Location header for ingest-by-reference fcrepo/fcrepo-specification#236 — require support for Content-type=message/external-body; access-type=URL (with a note that an impl may copy, cache, ingest, etc. if it wants to)
• Require support for external content with access-type=URL fcrepo/fcrepo-specification#237 — ingest-by-reference using the Location header to specify the remote content location.