Fix a couple small bugs in zerofetch.py

1) The deployed version of the requests module in precise is
old, so we need to use resp.content rather than resp.text
2) In a previous patchset when we switched to filecmp we
lost our existence check on the final output filename, so
that's been put back in the condition

Change-Id: I41e3e031e272e1cd505d11787b73682cb19f6520