Check for non-string exit code after `wp post edit`

Fixes bug introduced in f592cff, where quitting out of the system editor
would erase a post, because the `content_save_pre` filter would
fail to interpret system exit codes, and treat them as an empty string.

There must be a way to write a test for behavior like this, but how to
actually do that is beyond my knowledge. I did test with several
different $EDITOR settings (vi/nano/gedit), though, and this worked each
time for me.
wp post edit should no longer call parent::update

This broke in f6285d3 when CommandWithDBObject::update was removed and
_update made protected. It should have been defined as $this->update
from the beginning.
Add feature tests for `wp post edit`

Adds two new feature tests for wp post edit: one that simulates opening
an editor and quitting right out of it, and one that simulates chnaging
a word and then saving.
Merge pull request #801 from goldenapples into master