The automatic ETag generation we've had for a long time has been pretty
badly broken, and this became noticeable once we turned on automatic
ETags for all resources in Review Board recently.

Basically, we were computing an ETag from the available field
information on the resource, and not from the values for that particular
object. This meant that if a value changed, the ETag would not.

We now generate it based on the representation of each value in the
computed list.

Unit tests pass.
Tested this manually by fetching a resource, updating a value, and fetching

again.