Fix form handling for user feedback

issue was noticed on demo site, but possibly existed elsewhere?  also not 100%
sure about this fix, but it's believed to be okay...

tailbone/forms/core.py

@@ -918,16 +918,13 @@ class Form(object):
             if self.request.method != 'POST':
                 return False
 
-            # we assume data is in the form of POST params
-            controls = self.request.POST.items()
-
-            # but if not, we can try to parse request body as JSON
-            if not controls:
-                try:
-                    controls = self.request.json_body.items()
-                except ValueError:
-                    # not a valid JSON body, so no data
-                    controls = []
+            # use POST or JSON body, whichever is present
+            # TODO: per docs, some JS libraries may not set this flag?
+            # https://docs.pylonsproject.org/projects/pyramid/en/latest/api/request.html#pyramid.request.Request.is_xhr
+            if self.request.is_xhr:
+                controls = self.request.json_body.items()
+            else:
+                controls = self.request.POST.items()
 
             dform = self.make_deform_form()
             try: