Fix update after alter table adding a blob type.
(this needs to be rebased changing also the tests numbers if #92 lands before).
This is similar to #68 (closed) but it fixes a panic after adding a blob type (also time type and every other type saved as a blob).
Note: if using the provided test without the fix, the tests will enter an infinite loop in ql.go:1869 due to pc != db.cc. Now I haven't looked why this happened. Looks like after panicking a commit or another operation is done causing bad behavior. Additionally aren't pc and db.cc pointers to TCtx and the equality check should be done between their referenced values? (*pc != *db.cc)?