Matplotlib implicit converter registration not warning

In https://github.com/pandas-dev/pandas/pull/18307 we added something that should warn on

```python
import pandas as pd
import matplotlib.pyplot as plt
fig, ax = plt.subplots()
s = pd.Series(range(12), index=pd.date_range('2017', periods=12))
ax.plot(s)
```

but it's apparently not warning for me.