`push_resource` doesn't update an existing resource's bases

[Alex Lowe]

(Note: this behaviour could be intentional. If so, the ask here is for a brief note in the documentation just so it's unambiguous to future API users.)

This affects the `push_revision` endpoint on Charmhub: http://api.charmhub.io/docs/default.html#push_revision

To replicate:

1. Upload and push a new resource revision with one set of `bases`
2. Do the same again with the same file (thus duplicating the resource), but with a modified `bases`

Expected:

Resource bases get modified(?)

Actual:

Resource remains unmodified and final status is "approved" (no way to distinguish without further calls)