Fix wrapping context on named arguments.

[foobarna]

No description provided.

[davecoates]

I encountered this when trying to use procrastinate - deferring a task would hit an error about mixed arguments. This patch seems to work well. The error in question was:

django.db.utils.ProgrammingError: positional and named placeholders cannot be mixed

[wesleykendall]

thank you! I will see if I can write a test case for this for additional measure before releasing