Handle BLOBs with null terminators in them

(Cf #28) Some blob columns in SQLite get detected as text columns and then the resulting
data can be truncated due to the presence of null terminators. This change
checks to make sure that the byte length as specified by SQLite is the same as
the grabbed string.

I'd write a test for this, but (1) the tests weren't passing on master already (and I couldn't figure out how to fix them), and (2) I couldn't figure out how to insert a blob through SQLite.jl with null terminators in it a task for another day perhaps.