Add white-space: nowrap; to .dropdown-header

Dropdowns adjust their width to the width of their containing a elements - which have no line breaks ([*](https://github.com/twbs/bootstrap/commit/f226463b057f945795661c3d19fbcabf63b931f7)). Consistent to this dropdown-headers also should be prevent to contain line breaks.
Merge pull request #13851 from ahoereth/patch-1

Add white-space: nowrap; to .dropdown-header