How to use ``astropy.io.votable`` to fix non standard-compliant tables?

@mdboom - if I download the following VO table I get some warnings:

```
In [3]: t = Table.read('http://mpia.de/~robitaille/astropy4herts/notebooks/data/rosat.vot')
Downloading http://mpia.de/~robitaille/astropy4herts/notebooks/data/rosat.vot
|=============================================================================================| 5.4M/5.4M (100.00%)        19s
WARNING: W49: None:924:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:2170:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:22470:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:23884:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:27160:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:47362:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:48580:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:56322:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:56784:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:58086:6: W49: Empty cell illegal for integer fields. (suppressing further warnings of this type...) [astropy.io.votable.exceptions]
```

Now if I write this table to a new VO table:

```
 t.write('test.vot', format='votable')
```

then open a new IPython session and read it in again:

```
In [2]: t2 = Table.read('test.vot')
WARNING: W49: None:924:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:2170:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:22470:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:23884:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:27160:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:47362:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:48580:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:56322:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:56784:6: W49: Empty cell illegal for integer fields. [astropy.io.votable.exceptions]
WARNING: W49: None:58086:6: W49: Empty cell illegal for integer fields. (suppressing further warnings of this type...) [astropy.io.votable.exceptions]
```

I get the same warnings. Is there a way to fix these issues?