= Improvements

* The jdbc/h2 subadapter now supports H2 version 2.0.  It continues to
  support H2 versions 1.3 and 1.4.

* The mysql2 adapter's prepared statement support now reuses existing
  native prepared statements, instead of only binding variables on
  newly prepared statements.  This was the intended behavior
  previously, and should result in increased performance in cases
  where preparing a query takes significant time.

* The subclasses plugin now ignores an existing Class#subclasses
  method if it is defined in Ruby.  This fixes cases where usage of
  ActiveSupport would break the subclasses plugin.

* Database#call_sproc in the jdbc adapter will now always close the
  prepared call it creates.  Before, if there was an exception raised
  when setting the arguments for the prepared call, the prepared call
  would not be closed.

* A more appropriate error is now issued if you try to use the
  column_encryption plugin to encrypt a column without setting up an
  encryption key.
