Spawn new ResourceProxy for each chain so it has its own context

We never know when any part of a chained request (and its ResourceProxy) will
be saved to a variable, we create a new ResourceProxy for each refinement, so
fetches may be made individually, and further chaining will be segregated.

- Introduced spawning for all the refinement methods
- Wrote rdoc to all the methods in ResourceProxy
- Moved ResourceProxy tests from refinements_spec to resource_proxy_spec
- Organized tests into groups, added tests for spawning
- Added documentation explaining how chaining context works