modify get_value method for get multi-value

Because dictionary is queryDict,
If you send multiple values with the same field_name,
When using the '__get__' method, only the last element is reflected.

Use the 'getlist' method to return a list with multiple values

rest_framework/fields.py

@@ -422,7 +422,7 @@ class Field(object):
                 if getattr(self.root, 'partial', False):
                     return empty
                 return self.default_empty_html
-            ret = dictionary[self.field_name]
+            ret = dictionary.getlist(self.field_name)
             if ret == '' and self.allow_null:
                 # If the field is blank, and null is a valid value then
                 # determine if we should use null instead.