Front-end membership signup is broken - no membership record created

NOTE: I think this is a pretty recent regression - no idea what change triggered it Back-office signups are working correctly.

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To recreate:

• Use online contribution page with a membership block (sample page id=2 OR create a new page) to signup for a membership
• Transaction completes without error or notices but NO MEMBERSHIP record is created, and also NO MEMBERSHIP SIGNUP ACTIVITY.

I believe part of the problems is that the line_item row is being created incorrectly for front-end. After the changes in https://issues.civicrm.org/jira/browse/CRM-14197, membership related line_item rows SHOULD have:
entity_table = civicrm_membership
entity_id = civicrm_membership.id
contribution_id = id of the linked contribution

In front-end tests, this isn't happening. The entity_table is still civicrm_contribution and entity_id AND contribution_id are both the same (id of contribution record).

Screenshot shows 2 line items. Top line_item (orange highlight) was created by front-end signup. Second line_item was created by back-end signup - and is correct.