Fix file uploads in edit form

I've removed explicit `request.form` passing to edit form in the form instance
creation. The reason for this is that if we pass `formdata` to the constructor
it considers that all the data it needs, whilst `request.form` contains no
information about uploaded files.

If we simply omit passing `request.form` around, Flask-WTF takes care about
filling the form with `request.form` as well as `request.files`, if there are
any.

Hence file uploads now work in the model edit.

flask_admin/model/base.py

@@ -467,7 +467,7 @@ class BaseModelView(BaseView, ActionsMixin):
         """
         return self.get_form()
 
-    def create_form(self, form, obj=None):
+    def create_form(self, form=None, obj=None):
         """
             Instantiate model creation form and return it.
 
@@ -475,7 +475,7 @@ class BaseModelView(BaseView, ActionsMixin):
         """
         return self._create_form_class(obj=obj)
 
-    def edit_form(self, form, obj=None):
+    def edit_form(self, form=None, obj=None):
         """
             Instantiate model editing form and return it.
 
@@ -821,7 +821,7 @@ class BaseModelView(BaseView, ActionsMixin):
         if not self.can_create:
             return redirect(return_url)
 
-        form = self.create_form(request.form)
+        form = self.create_form()
 
         if form.validate_on_submit():
             if self.create_model(form):
@@ -854,7 +854,7 @@ class BaseModelView(BaseView, ActionsMixin):
         if model is None:
             return redirect(return_url)
 
-        form = self.edit_form(request.form, model)
+        form = self.edit_form(obj=model)
 
         if form.validate_on_submit():
             if self.update_model(form, model):