PHPBrowser should contains methods for grabbing HTTP headers

[zdenekgebauer]

Module REST contains methods for grabbing HTTP headers (grabHttpHeader, seeHttpHeader...). I am convinced that module PHPBrowser should contains these methods, because common pages also can have some special headers (CSP, X-Frame-Options, Feature-policy etc.)

Solution for now is combination of modules PHPBrowser and REST in one test suite, but for common pages without any API it complicates configuration.