Output a string bind type for selects

[yanokwa]

Closes #168

[lognaturel]

I am running the failing test with nosetests pyxform.tests.xform2json_test.DumpAndLoadXForm2JsonTests. It looks like SurveyElement._link_children is hitting a case where something that was previously a SurveyElement is now a dict. I thought it might have something to do with the presence of or_other but I think this affects any select.

For example, in the specify_other.xls case, I see the following json generated for the male choice when the tests pass:
{'name': 'male', 'compact_tag': '', 'sms_field': '', 'sms_option': '', 'label': {'English': 'Male'}, 'hint': '', 'guidance_hint': '', 'default': '', 'type': '', 'appearance': '', 'parameters': '', 'intent': '', 'jr:count': '', 'bind': {}, 'instance': {}, 'control': {}, 'media': {}, 'parent': None, 'children': [], 'itemset': '', 'choice_filter': '', 'query': '', 'autoplay': '', 'flat': False, 'action': '', 'list_name': ''}

This suggests to me that select choices are being represented as SurveyElement objects which do have children attributes.

When the tests fail, select choices are represented as dictionaries:
{'name': 'male', 'label': {'English': 'Male'}}

I don't know why a select choice would be represented as a SurveyElement or why that json output would be desirable. I don't believe that any of the SurveyElement attributes could be populated.

XForm2json support is experimental and as far as I know, it is unused. There are three forms in that test that contain selects and thus result in the failure: specify_other.xls, loop.xls, xlsform_spec_test.xlsx, yes_or_no_question.xls

[lognaturel]

The XForms to json functionality was using the bind type rather than the body type to identify selects. I have added a commit to override this. I have run nosetests locally and confirmed they pass. I have also run black pyxform.

I fully expect that some other downstream integrations use the bind type this way. We just need to be ready to react when folks run into issues.