information schema table.

The get_schema_views_record() function fills records in the view table of
the informations schema with data about given views. Among other info
the is_updatable flag is set. But the check whether the view is updatable or
not wasn't covering all cases thus sometimes providing wrong info.
This might led to a user confusion.

Now the get_schema_views_record function additionally calls to the 
view->can_be_merge() function to find out whether the view can be updated or
not.


mysql-test/t/view.test:
  Added a test case for the bug#30020: Insufficient check led to a wrong info
  provided by the information schema table.
mysql-test/r/view.result:
  Added a test case for the bug#30020: Insufficient check led to a wrong info
  provided by the information schema table.
sql/sql_show.cc:
  Bug#30020: Insufficient check led to a wrong info provided by the
  information schema table.
  Now the get_schema_views_record function additionally calls to the 
  view->can_be_merge() function to find out whether the view can be updated or
  not.