We use as URLMapTransport the test transport in our unit tests, which

lets callers easily define response payloads for our API URLs in unit

tests.
While writing some unit tests, I found that I couldn't reliably update

the root resource's payload. I could update the payload returned by the

root URL endpoint itself, but the server info resource and capabilities

attribute on the class would still use information from the default root

resource that gets set during URLMapTransport.initialize(). This

change allows callers to more easily change the root resource, and

dynamically reflect those changes in associated resources and attributes.
This change also fixes two bugs:


We weren't setting the right object representation for the server info

   resource. We were setting it to the outer payload when it should just

   be the inner payload (e.g. the object representation, without the

   'item_key' and 'stat' info).


We weren't handling object representations that were callables. We

   do handle payloads that are callables, but it's possible to have

   object representations inside the payload that are callables too

   (as is the case for the server info resource). We now process these

   like we do for callable payloads.

Used this change in an upcoming one that has unit tests that

  previously crashed without these fixes.
Ran unit tests.