Pass HTTP client instances through when creating children

[acdha]

I have a project which uses an API which returns relative URLs and requires HTTP authentication and at times access via a proxy. This has exposed several cases where the HTTP client wasn't being passed to a child object, which would cause the initial get() to work and then subsequent accesses to fail.

acdha/remoteobjects@746ce0b passes the client to Link
acdha/remoteobjects@aa1eaf1 passes the client in to a RemoteObject during post()

The major remaining area is handling collections: currently it's complicated by the fact that a List or Dict doesn't pass a reference to itself in when creating a subfield so the fix will need to pass a reference to self or self._http from ListObject.seqmethod through into the underlying field decode. I'm currently handling this with a quick hack to fix it up on the way out of getitem but it seems like this will require changing field decode() to accept an optional http parameter to pass the value down.

[acdha]

An experimental test implementation is in acdha/remoteobjects@1c8c820 - see https://github.com/acdha/remoteobjects/tree/issue-12-linked-clients. It still needs tests to confirm that the same HTTP client is being passed around everywhere but it appears to work in casual testing.

[acdha]

We have been running the attached code w/o issue for around a bit over a year without incident

[samv]

Hi Chris, would you be able to write at least a couple of tests to demonstrate this? I'm just starting to look at some of the old pull requests and seeing some tests would help clarify the intent of this change.