Test for From_ followed by valid header in bin/cleanarch is incomplete.

Part of the logic in bin/cleanarch in determining if a From_ line is a valid Unix From_ separator checks if the following line is a valid header. This test is intended to be that the following line contains a colon and only one or more ascii printable characters up to the first colon. It does check for a colon, but only checks that there is at least one ascii printable character at the beginning of the line before the colon.