INTPYTHON-348 Add support for QuerySet.raw()

- Fix __str__ on MongoRawQuerySet

Author: aclark4life

django_mongodb/query.py

@@ -320,6 +320,7 @@ def raw_mql(self, raw_query, params=(), translations=None, using=None):
 
 
 class MongoRawQuery(RawQuery):
+
     def __init__(self, sql, using, model, params=()):
         super().__init__(sql, using, params)
         self.model = model
@@ -343,6 +344,9 @@ def get_columns(self):
         converter = connections[self.using].introspection.identifier_converter
         return [converter(column_name) for column_name in self.model._meta.fields]
 
+    def __str__(self):
+        return "%s" % self.sql
+
 
 class MongoRawQuerySet(RawQuerySet):
     def __init__(