RBTools's HTTP caching is conditional based on the version of Review

Board. In versions prior to 2.0.14, caching was broken in various ways,

so RBTools only enables it if the root resource claims a newer version.

By default, though, caching was disabled, meaning the root resource

never went through the caching support.
Since most companies are using versions far newer than the minimum of

2.0.14, this change flips the logic around. Caching is now enabled by

default for all resources, and is then disabled if the root resource

claims an earlier version. This gives us the best performance available

for most users.
Some API transport and response testing infrastructure had to be updated

to test the new code paths. Those are included in this change.

Unit tests pass.
Tested posting changes against reviews.reviewboard.org and local

servers.