assuming title case is good for headers is too bold

Currently the `verbose_name` of columns is converted to title case. This makes sense if the `verbose_name` is taken from the python attribute name, but when it was specified explicitly it should be left alone.

This can be worked around by using `mark_safe`, however if the verbose name comes from a translation, we cannot safely assume that it doesn't contain malicious code.

If you agree I will prepare a PR to this effect.