Fix database routing bug on save()

[bckohan]

Closes #865

This bug is a regression introduced in 4.11 when the PolymorphicModel.save() arguments were typed.

Prior to 4.11 the code passed the arguments through using **kwargs. The regression was caused by using being set to if it was none - but the router was not consulted first.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.24%. Comparing base (28994d3) to head (03314a4).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #866      +/-   ##
==========================================
- Coverage   91.25%   91.24%   -0.01%     
==========================================
  Files          28       28              
  Lines        1899     1897       -2     
  Branches      274      273       -1     
==========================================
- Hits         1733     1731       -2     
  Misses        110      110              
  Partials       56       56              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.