Better handling for broken MIME structure

[rjl]

The display_parts() function in mime.php tries its best to render the MIME structure of the e-mail message it's passed, but in some pathological cases the MIME structure itself is broken, and cannot be properly parsed. This results in errors, such as:

Warning: htmlentities() expects parameter 1 to be string

This can happen, for instance, when there are multiple "To:" headers, such that $structure->headers["to"] is seen as an array rather than a simple string.

[dmorton]

Confirmed still broken. Of course, how does one properly view a message with bad headers or bad MIME structure?

[mortonda@…]

[1339],[1340] just make the output loop over the values. It happens to do the right thing for the normal case. Also, #517 changed the location of the impact of this ticket.