fix issue with GET on large object

Fixing a call to _drop_cache that had an extra parameter

A patch to this issue was first provided by Xw Huang(xwhuang@qnap.com)
in gluster-swift gerrit (http://review.gluster.org/#/c/7730).
I'm transferring the patch to swiftonfile.

Fixes issue #13

Signed-off-by: Thiago da Silva <thiago@redhat.com>