backport: woob.browser.url: add more matching options

• Add the headers property and URL.with_headers method, to automatically add request headers on URL.go or URL.open.

class MyBrowser(PagesBrowser):
    BASEURL = 'https://my-super-api.example/for-tpps/'

    my_page = URL(r'my-super-path', MyPage, headers={
        'Accept': 'application/vnd.my-super.api+json; version=4',
    })

    def my_method(self):
        # The headers will be added automatically by the URL here!
        self.my_page.go()

• Add the URL.with_base method for being able to change the base.

class MyBrowser(PagesBrowser):
    my_page = URL(r'my-super-path', MyPage)

class MyChildBrowser(MyBrowser):
    BASEURL = 'https://my-super-api.example/main/'
    ALT_BASEURL = 'https://my-super-api.example/alt/'

    my_page = MyBrowser.my_page.with_base('ALT_BASEURL')

• Add methods to URL matching, for e.g. pages that are different / have different roles between GET and POST.

class MyBrowser(PagesBrowser):
    BASEURL = 'https://my-super-api.example/main/'

    my_collection_page = URL(r'my-collection', MyCollectionPage, methods=('GET',))
    my_new_element_page = URL(r'my-collection', MyNewElementPage, methods=('POST',))

• Add content_type to URL matching, for e.g. pages that are different depending on the Content-Type header.

class MyBrowser(PagesBrowser):
    BASEURL = 'https://my-super-api.example/main/'

    my_xml_resource = URL(r'my-resource', MyXMLResourcePage, content_type='application/xml')
    my_json_resource = URL(r'my-resource', MyJSONResourcePage, content_type='application/json')

    def my_method(self):
        self.my_xml_resource.go()
        self.page.get_resource_id()  # Will behave differently based on JSON or XML.

• Refactor / simplify URL tests (to not use a class).