TST: Test for failing case.
BUG: Allow forecasting with trend=nc. Closes #1123.
ENH: Don't require in-sample exog for forecast.
TST: forecast checks for 2d exog now.
TST: Adjust precision for local failure.
ENH: Make sure exog is given for each step in ARMAX forecast.
TST: Test trend=c and 1 exog variable.
ENH: Make 1d exog conformability check robust for predict.
TST: No longer need in-sample exog values for forecast.
Merge pull request #1124 from jseabold/fix-1123

BUG: Fix ARIMA prediction when fit without a trend.