Previews don't work because the snapshot XML is broken

[jnm]

NOTE: use this branch, which now has a failing test, when working on this issue: https://github.com/kobotoolbox/kpi/tree/2029-snapshot-xml-renderer

Can’t preview any Enketo form; Enketo responds with an error: Extra content at the end of the document.

Adding rest_framework_xml.renderers.XMLRenderer to settings.REST_FRAMEWORK['DEFAULT_RENDERER_CLASSES'] has caused our custom XML renderers not to be used:

kpi/kpi/renderers.py

Lines 22 to 39 in 78e5ab0

	class XFormRenderer(renderers.BaseRenderer):
	media_type = 'application/xml'
	format = 'xml'
	charset = 'utf-8'
	def render(self, data, media_type=None, renderer_context=None):
	asset = renderer_context['view'].get_object()
	return asset.snapshot.xml
	class AssetSnapshotXFormRenderer(renderers.BaseRenderer):
	media_type = 'application/xml'
	format = 'xml'
	charset = 'utf-8'
	def render(self, data, media_type=None, renderer_context=None):
	asset_snapshot = renderer_context['view'].get_object()
	return asset_snapshot.xml

See

kpi/kpi/views.py

Lines 598 to 600 in 78e5ab0

	renderer_classes = NoUpdateModelViewSet.renderer_classes + [
	AssetSnapshotXFormRenderer,
	]