Tweak how we detect JSON request body instead of POST params

i.e. when processing submitted form data

tailbone/forms/core.py

@@ -921,7 +921,7 @@ class Form(object):
             # 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:
+            if self.request.is_xhr and not self.request.POST:
                 controls = self.request.json_body.items()
 
                 # TODO: why in the hell is this necessary?  some colander forms