Ensure that tuples are not considered as data, not as data containers

It can happen (not necessarily by craziness, but because of default names given by pandas.core.groupby.DataFrameGroupBy.agg() ) that names of columns are tuples. If one tries to print a regression summary() in such cases,
1) if the column is used as a dependent variable, there will be an error,
2) if the column is used as an independent variable, "%s" will be printed instead than its name.

My change ensures that tuples are considered as objects for interpolation, not as containers of objects. It should have no effect at all on any data type which is not a tuple.

BTW: tested only on d5ed746752ed323208171a21d104e75fc287de48 , unfortunately later commits give me an error ("ImportError: cannot import name kalman_loglike", at line 32 of statsmodels/statsmodels/tsa/kalmanf/kalmanfilter.py ). But I did check that the file was not changed by any subsequent commit.