This turned out to actually make testing more difficult, as the
FileStore objects are generally mocked themselves and moving the Close()
call inside them introduced IO problems in the test suite.
This reverts commit a063f85eca5dd3f983bc9b790fd68108820cc730.
Accepting a ReadCloser in place of a Reader allows the FileSystem
implementation to handle closing the reader, which in turn simplifies
downstream code.
