TST: Test missing failure with formula.
BUG: combined can be empty if patsy handles missing. Closes #2083.
TST: Test that missing, formula, and extra arrays work.
ENH: Ensure extra arrays are attached as ndarrays.
BUG: Check input array lengths after missing data handling.
TST: Test that missing, formula, and extra arrays work.
BUG: Check input array lengths after missing handling.
TST: Test for extra arrays with missing and formula
BUG: Handle 2d extra arrays and endog/exog with formula and missing.
TST: Update for dimension handling higher up.
ENH: Ensure ndarray-like is passed up. Needs refactor.
Merge pull request #2084 from jseabold/fix-2083

BUG: Correct issue if patsy handles missing. Closes #2083.