test($http) - add tests for functional header values

Closes #5784

fix($http) - run execHeaders on merged header object

fixes #5784

fix($http) - remove unnecessary execHeaders call

The defHeaders object is merged into the reqHeaders object. The
combined object is then passed into execHeaders which evaluates all
header functions.

test($http) - simplify testing of default header functions

Remove unneeded 'should override default headers with header function'.