Prevent property with trailing equal sign in config set

[sbesson]

See https://www.openmicroscopy.org/community/viewtopic.php?f=5&t=7752#p15430

This PR should prevent an operation like

bin/omero config set omero.property= value

Apart from checking the unit tests are green in Travis, testing this PR should include running this command and testing config.xml remains unmodified.

--no-rebase

[manics]

Is there a specification for allowed characters in keys? E.g. if it's limited to [A-Za-z0-9\.]+ could you do a more thorough check?

[joshmoore]

There's not really anything that's filtered. This is mostly a usability check in terms of "a=b" is probably not what you mean, since that means "remove the key named 'a=b'". If we added "bin/omero config unset a=b" and deprecate the one arg usage, perhaps this would get easier.

[manics]

Good to merge then.