.ics export violates RFC2445

[Adam Spiers <orgmode@adamspiers.org>]

I use org-export-icalendar-combine-agenda-files to export my
appointments to an .ics file which I point korganizer at.

I noticed ages ago that if I have an appointment with a comma in, e.g.:

** <2007-12-07 Fri 20:00> foo, bar

korganizer always shows it as "bar" rather than "foo, bar".  But I
never got round to investigating whether it was a bug with the export
or korganizer or something else ... until now :-)  I just took a quick
look at the iCalendar spec, which is RFC2445, and discovered that the
SUMMARY field is defined as follows

   summary    = "SUMMARY" summparam ":" text CRLF
 
    -- from http://tools.ietf.org/html/rfc2445#section-4.8.1.12

And the definition of 'text' in this context explicitly states that
several characters, including commas, need to be escaped with a
backslash:

   http://tools.ietf.org/html/rfc2445#section-4.3.11

Sure enough, when I edited the .ics file and manually escaped the
comma, korganizer displayed the summary correctly.