fix padding error

[hhaensel]

This PR addresses #121
Not sure, what a good way of warning could be. Currently I spit out a warning every time, which is probably not the best idea for the final version.
Mikmoore's comments on discourse were very helpful for finding this solution.

[hhaensel]

I also tried unsafe_wrapping for types containing arrays, like Dates.CompoundPeriod, which cannot be written with this solution.
Interestingly, that worked, although I thought it should not. Will investigate deeper. But that's nothing for this PR.

EDIT: I think I misinterpreted an error message and couldn't reproduce, so please ignore this post.

[hhaensel]

@DhairyaLGandhi not sure whom to ask for a review, could you help?