BUG: mssql.TIMESTAMP -> bytes_ breaks discover

Hello,

I noticed from [this commit](https://github.com/blaze/odo/commit/5c8232bf7eb73cca98e32cff3acc4fb18f4dcf18#diff-51da4c3b85f1cc5241513828c9882880R104) that I think we accidentally broke datetime discovery in blaze.

Just try this code after this commit:

```python
import sqlalchemy as sa
from odo import discover
columns = [sa.Column('amount', sa.REAL),
sa.Column('ds', sa.TIMESTAMP)]
dta = sa.Table('t', sa.MetaData(), *columns)
discover(dta)
```

this returns `dshape("var * {amount: ?float32, ds: ?bytes}")`. Note that 'ds' is now `?bytes`, not `timestamp`...

(This breaks unittests in blaze too... trying to get those back)

@postelrich @dhirschfeld @llllllllll