Revert "Let FitsHeader take an unnamed file_name argument in addition to the kwarg version.  (Basically interprets header as file_name if it is a string)"

This reverts commit 5095311eb14a1e11f27401c55acce3adb2655dfc.
Let FitsHeader take an unnamed file_name argument in addition to the kwarg version.  (Basically interprets header as file_name if it is a string)
(#508)
Enable reading FitsHeader from .head files with text_file=True kwarg
(#508)