box: fix inheriting format options for old-style parts

If index parts are specified using old syntax like:

	parts = {1, 'number', 2, 'string'},

then (except if parts count is 1) index options set in space format
are not taken into account. Solution is to continue after parsing 1.6.0
style parts so to use code that check format options.

Closes #7614

NO_DOC=bugfix