The files.com integration tests for rcat/copyurl were failing because
fs/account.Account was declaring a ReadAt method when the underlying
handle did not support it. The files.com SDK decided to use the ReadAt
method to speed transfers up which failed.
ReadAt and Seek methods were added in this commit to support the
archive command:
409dc75328 accounting: add io.Seeker/io.ReaderAt support to accounting.Account
This fixes the problem by adding new methods to the Account object
WithSeeker/WithReaderAt/WithReadAtSeeker which produce an object with
the desired methods or errors if it isn't possible.
This stops Account advertising things it can't do which is bad Go
practice.
