Failed sharing checked incorrectly

Share return was checked incorrectly:

HTTP Status: 403 does not contain not allowed to share, pyocclient allows accessing the body of the message which we compare now with f6412ae
Made the permission check to check for the required permission, to allow easier adding of new test cases 507775f
Check that the owners file is not modified when editing is not allowed e778236

Fix owncloud/smashbox#82 Cherry-picked from owncloud/smashbox#83

@moscicki

cernbox / smashbox