fix missing multiple values for the same parameter name (#565)

caravel/viz.py

@@ -21,7 +21,7 @@
 from markdown import markdown
 import json
 from six import string_types
-from werkzeug.datastructures import ImmutableMultiDict
+from werkzeug.datastructures import ImmutableMultiDict, MultiDict
 from werkzeug.urls import Href
 from dateutil import relativedelta as rdelta
 
@@ -110,12 +110,13 @@ def get_url(self, **kwargs):
             del d['action']
         d.update(kwargs)
         # Remove unchecked checkboxes because HTML is weird like that
-        od = OrderedDict()
+        od = MultiDict()
         for key in sorted(d.keys()):
             if d[key] is False:
                 del d[key]
             else:
-                od[key] = d[key]
+                for v in d.getlist(key):
+                    od.add(key, v)
         href = Href(
             '/caravel/explore/{self.datasource.type}/'
             '{self.datasource.id}/'.format(**locals()))