Fix a couple issues with association cloning

This fixes two issues with association cloning.  The first issue
is that :eager_block was not cloned from the source dataset if
it wasn't set specifically via an option (the default is to use
the block passed to the method).

The second issue is that cloning an association that was a clone
of another association didn't have the block from the original
association.
Failing spec when limiting datasets that include clob columns in the oracle adapter.
Make filter by associations support handle associations with :conditions or block

This fixes a documented limitation with the filter by associations
support, where it previously did not handle associations that
were not entirely specified by the keys.  This allows the filter
by associations to work with associations with :conditions and
most type of blocks, by adding an extra subselect that tests
that the object(s) used as the filter by associations value also
meet the criteria for the association.
Merge branch 'master' of git://github.com/sdeming/sequel
Make offset emulation on Oracle work when using columns that can't be ordered (Fixes #724)

This changes Oracle from using the generic ROW_NUMBER window
function emulation to using a specific ROWNUM emulation. The
generic ROW_NUMBER window function can add orders on all columns,
and Oracle doesn't allow you to order on some column types.

Fix the oracle spec added to test this, since it tests for
ordered output without specifying an order, and that's not safe,
as without an order specified, the database is free to return
rows in any order.
Add note to serialization plugin about serialization_modification_detection (Fixes #726)