Floating tripwire disappears upon upgrading

Put the summary of the bug you're having here

Upon upgrading from an old world with floating string, all floating string that is loaded disappears without dropping. (Likely caused by removal of distinction between "floating" and "supported" tripwire.)

What I expected to happen was...:
When upgrading to a newer snapshot, all tripwire should convert to the new form.

What actually happened was...:
When upgrading to the newer snapshot, all tripwire simply vanished.

Steps to Reproduce:
1. Create a world in 15w47c or earlier (1.8, 1.7...)
2. Place some floating string (does not matter if attached to tripwire hooks or not)
3. Upgrade to 15w49a or later
4. All floating string will have disappeared

I should also mention that when downgrading back to 15w47c or earlier, any new floating string will count as supported string, and will pop off if updated.