Check that the form input element has given value.

def input_has_value(step, field_name, value):
    """
    Check that the form input element has given value.
    """
    with AssertContextManager(step):
        text_field = find_any_field(world.browser,
                                    DATE_FIELDS + TEXT_FIELDS,
                                    field_name)
        assert_false(step, text_field is False,
                     'Can not find a field named "%s"' % field_name)
        assert_equals(text_field.get_attribute('value'), value)