fix x property assignment

add_x_property works fine. But, x_foo = 'bar' does not.
adjust method_missing handling.
make x properties act as text property

+ event.x_foo should return decoded string (Ruby's String)
  as event.summary does. RFC2445-encoded string is aquired
  by event.x_properties['X-FOO'].to_s.

+ Remove extra colon of x-properties when exported to string.