BUG: Exception when setting a major- or minor-axis slice of a Panel with RHS a DataFrame
Fixes GH #11014

This isn't actually a recent regression. The bug was introduced in 0.15.0 with this commit: https://github.com/pydata/pandas/commit/30246a7d2b113b7115fd0708655ae8e1c3f2e6c5; it was just made more common (i.e., it happens on panels without mixed dtype) in 0.16.2 by this commit: https://github.com/pydata/pandas/commit/5c394677e10dac38fe3375e2564bb76cf8bbffca.

@evanpw gr8 thanks!
