tests: modify command doesn't need --acl-public anymore

The modify command now preserves the original ACL, so we don't need to
specify --acl-public anymore to have it have the proper ACL.
[modify] only change mime-type if it's passed on the command line

or if expliclty put into the config file (but that would be odd).
tests: [modify] add and remove cache-control header

This tests the --add-header and --remove-header functionality.

This also adds a dependency on curl to run these tests.