Fixed problem with bulk export failing

http://manage.dimagi.com/default.asp?88314
Is `self.exportFilters` a full URL with query string? It appears to be just a string of URL-encoded parameters from looking at places where it is passed into ExportManager:

[case_export_data.html:17](https://github.com/dimagi/commcare-hq/blob/master/corehq/apps/reports/templates/reports/reportdata/case_export_data.html#L17)
[excel_export_data.html:17](https://github.com/dimagi/commcare-hq/blob/master/corehq/apps/reports/templates/reports/reportdata/excel_export_data.html#L18)
[export.py:73](https://github.com/dimagi/commcare-hq/blob/master/corehq/apps/reports/standard/export.py#L73)

Would be nice to avoid the parameter encode and then this subsequent manual decode. Maybe it would make sense to change the places where ExportManager is instantiated, and pass in a JS object (dict) rather than a string?

```diff
-            exportFilters: '{{ get_filter_params.urlencode|safe }}',
+            exportFilters: {{ get_filter_params|JSON }},
```

I'm not sure if you will need to change the definition of [`get_filter_params`](https://github.com/dimagi/commcare-hq/blob/master/corehq/apps/reports/standard/export.py#L73) to return a dict. It looks like it's returning something similar to a dict, but it may not be able to be converted directly to JSON.