disambiguate broken because of unicode/str mix in mechRepr results

[icemac]

The following scenario error leads to an error:

• A page contains a submit control whose value contains a non-ASCII character.
• There is also a select with options or a text input field on this page.
• browser.getControl() is used to select a not existing control.

This leads to a UnicodeDecodeError when zope.testbrowser tries to render the list of possible controls using the mechRepr of the controls. The exception happens when joining the mechRepr values of the controls because to a single string. The ones of the select options and text fields are unicode while the one of the submit control is str but contains a non-ASCII char.