issue2550535 Fix keep_quoted_text=yes setting. In some cases even when keep_quoted_text=yes is configured we would strip quoted sections. This hit the python bug-tracker especially for python interpreter examples with leading '>>>' strings. The fix is slightly different compared to the proposal as this broke keep_quoted_text=no in certain cases. We also fix a bug where keep_quoted_text=no would drop the last line of a non-quoted section if there wasn't an empty line between the next quotes.

Structure of the tests:

   1   Test date classes
   1.1 Date
   1.2 Interval
   2   Set up schema
   3   Open with specific backend
   3.1 anydbm
   4   Create database base set (stati, priority, etc)
   5   Perform some actions
   6   Perform mail import
   6.1 text/plain
   6.2 multipart/mixed (with one text/plain)
   6.3 text/html
   6.4 multipart/alternative (with one text/plain)
   6.5 multipart/alternative (with no text/plain)