BUG: Fix failure to return monic polynomials from roots.

This bug affected the various polynomial class methods fromroots due to
the ability to specify both window and domain. In that circumstance the
roots are mapped from the domain to the window by the substitution
`x = off + scl*x`. The polynomial that was being generated was monic in
the window before substitution, but if scl was not one it was not monic
considered as a function of the variable x in the domain. The fix is to
divide the generated coefficients by `scl ** deg` so that the scaling of
the highest degree term after substitution is canceled.

It might be better to make the scaling optional in the future, but this
fix makes the result match the documentation.

Closes #3467.
Merge pull request #3495 from charris/make-fromroots-return-monic

BUG: Fix failure to return monic polynomials from roots.
BUG: Allow to pad arrays by zero entries.

np.pad will now accept a pad_width containing zeros. The functionality
was already implemented, but validation of input was too strict.
TST: New test to ensure np.pad allows pad_width of zero.
Merge pull request #3506 from josteinbf/arraypad-padwidth-zero

Allow pad_width of zero in np.pad