Problem: Regular users couldn't create requests while admins could
Root cause: to_internal_value() used dict(QueryDict) which converts
all values to lists, breaking validation
Solution:
- For QueryDict: use _mutable flag to modify in-place
- For regular dict: use dict() conversion as before
- Avoids ['value'] instead of 'value' conversion
This fixes 400 errors for non-admin users when creating requests
with multipart/form-data.
Игорь Надеин
d1b58d4c2d