Dataset export of a query requires a limit
I'm trying to download all of HGNC but with just 2 fields and following the instructions in the new developer docs.

```
In [9]: dataset = Dataset.retrieve('HGNC/3.0.0-2017-03-01/HGNC')

In [10]: query = dataset.query(fields=['gene_symbol', 'name'])

In [11]: export = query.export(format='csv',follow=True)
```

results in
`SolveError: Bad request. {u'limit': [u"'limit' must be between 1 and 1000000"]}`

It seems that like if I can download the entire dataset with just Dataset.retrieve().export, I should be able to do the same with a subset of the fields? 
The download works if I set
`query = dataset.query(fields=['gene_symbol', 'name'], limit=100000)`
diff --git a/solvebio/query.py b/solvebio/query.py
index e9d2384..82930c7 100644
--- a/solvebio/query.py
+++ b/solvebio/query.py
@@ -648,12 +648,17 @@ class Query(object):
     def export(self, format='json', follow=True, limit=None):
         from solvebio import DatasetExport
 
-        if not limit and self._limit < float('inf'):
-            limit = self._limit
-        params = self._build_query(limit=limit)
+        params = self._build_query()
         params.pop('offset', None)
         params.pop('ordering', None)
 
+        # if limit is not set use max limit
+        if not limit and self._limit < float('inf'):
+            limit = self._limit
+
+        if limit:
+            params['limit'] = limit
+
         export = DatasetExport.create(
             dataset_id=self._dataset_id,
             format=format,
['solvebio/query.py']