Allow body on HTTP DELETE view methods (#122)

* Allow body in delete requests
* Do not add request body to DELETE by default
* Check manual form parameters against body_methods
* Add tests
* Add changelog

Closes #118

src/drf_yasg/inspectors/base.py

@@ -276,7 +276,10 @@ class SerializerInspector(FieldInspector):
 
 
 class ViewInspector(BaseInspector):
-    body_methods = ('PUT', 'PATCH', 'POST')  #: methods that are allowed to have a request body
+    body_methods = ('PUT', 'PATCH', 'POST', 'DELETE')  #: methods that are allowed to have a request body
+
+    #: methods that are assumed to require a request body determined by the view's ``serializer_class``
+    implicit_body_methods = ('PUT', 'PATCH', 'POST')
 
     # real values set in __init__ to prevent import errors
     field_inspectors = []  #:

src/drf_yasg/inspectors/view.py

@@ -101,7 +101,7 @@ class SwaggerAutoSchema(ViewInspector):
             if isinstance(body_override, openapi.Schema.OR_REF):
                 return body_override
             return force_serializer_instance(body_override)
-        elif self.method in self.body_methods:
+        elif self.method in self.implicit_body_methods:
             return self.get_view_serializer()
 
         return None
@@ -144,8 +144,11 @@ class SwaggerAutoSchema(ViewInspector):
             raise SwaggerGenerationError("specify the body parameter as a Schema or Serializer in request_body")
         if any(param.in_ == openapi.IN_FORM for param in manual_parameters):  # pragma: no cover
             if any(param.in_ == openapi.IN_BODY for param in parameters.values()):
-                raise SwaggerGenerationError("cannot add form parameters when the request has a request schema; "
+                raise SwaggerGenerationError("cannot add form parameters when the request has a request body; "
                                              "did you forget to set an appropriate parser class on the view?")
+            if self.method not in self.body_methods:
+                raise SwaggerGenerationError("form parameters can only be applied to (" + ','.join(self.body_methods) +
+                                             ") HTTP methods")
 
         parameters.update(param_list_to_odict(manual_parameters))
         return list(parameters.values())