Evaluate the default-form only if we actually use it.

- New hash-table was being created with a bogus rehash-size.
- Added regression test in another patch.

	* hash-tables.lisp (copy-hash-table): Added new &key arg :KEY;
	it's run on each element before it's stored in the new HT.
	Additionally, make all &key args get their default even when
	NIL was passed for them.

	* tests.lisp (copy-hash-table.2): New test case.

 * new function: like GETHASH, but saves the default value in table if
   key is not found.*