KeyError with mongo backend and collection of variable documents

Hi,

I am using mongodb as backend.

I got these lines of code:

```
data_col = "mongodb://localhost/::"

import odo
resource = odo.resource(data_col)
dshape = odo.discover(resource)
odo.convert(list,resource)
```

unfortunately i end up wit a KeyError. Something like that

> 178 @convert.register(list, Iterator, cost=1.0)
> 179 def iterator_to_list(seq, **kwargs):
> --> 180 return list(seq)
> 181
> 182
> KeyError: u'search:seed'

The problem is that in my collection in mongodb not all documents has this keyword : `search:seed`

Even if I provide the correct dshape, I still get the same error.

The problem looks rather related to the use of

`toolz.itertoolz.pluck(ind, seqs, default='__no__default__')`

in this method

```
def _into_iter_mongodb(coll, columns=None, dshape=None):
""" Into helper function

Return both a lazy sequence of tuples and a list of column names
"""
seq = coll.find()
if not columns and dshape:
columns = dshape.measure.names
elif not columns:
item = next(seq)
seq = concat([[item], seq])
columns = sorted(item.keys())
columns.remove('_id')
return columns, pluck(columns, seq)
```

because we are not using the `default` option that would allow to deal with this case.