Don't use runInSeparateProcess

Directly calling "header" in the PHPUnit process causes the "Cannot
modify header information - headers already sent by" error to be thrown.
Instead of running the test in a separate process, which is slower, this
commit wraps the call to "header" in a method that can be mocked in the
tests.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>