ArvadosFileReader object always tries to return the exact amount of data asked for.

ArvadosFileReader object always tries to return the exact amount of data asked for.

This is to avoid breaking stuff like gzip that assumes it always gets the exact
amount of data unless EOF.  (From the Python file docs for read(): Note that
this method may call the underlying C function fread() more than once in an
effort to acquire as close to size bytes as possible.) closes #5856