Make add_pageblock_from_dict more flexible

This allows you to pass a pageblock instance to
`add_pageblock_from_dict`, as well as a dictionary.

For a complex custom pageblock, it can be simplest to initialize it
on its own with a factory, and then pass it into a call to
`add_child_section_from_dict`, similar to here:
http://django-pagetree.readthedocs.org/en/latest/testing.html

referencing issue #92

pagetree/generic/models.py

@@ -20,6 +20,13 @@ class Meta:
     display_name = None
     """The name for this block that's used in pagetree's menus."""
 
+    def as_dict(self):
+        """Returns the PageBlock as a dictionary.
+
+        :rtype: dictionary
+        """
+        return dict()
+
     def allow_redo(self):
         """Determines whether this block allows users to resubmit answers.
 

pagetree/models.py

@@ -538,7 +538,14 @@ def from_dict(self, d):
             self.add_child_section_from_dict(c)
 
     def add_pageblock_from_dict(self, d):
-        target_type = d.get('block_type', '')
+        try:
+            target_type = d.get('block_type', '')
+        except AttributeError:
+            # If we can't call .get on the dictionary,
+            # it must not be a dictionary. Assume it's a pageblock,
+            # and turn it into a dict here.
+            d = d.as_dict()
+            target_type = d.get('block_type', '')
 
         # now we need to figure out which kind of pageblock to create
         found_pbclass = None

pagetree/test_models.py

@@ -67,7 +67,9 @@ def edit(self, vals, files):
         self.save()
 
     def as_dict(self):
-        return dict(body=self.body)
+        d = super(TestBlock, self).as_dict()
+        d['body'] = self.body
+        return d
 
     def import_from_dict(self, d):
         self.body = d.get('body', '')