Using addDetail() is fiddly for simple cases

I want to be able to write, for example:

    with ExpectedException(Exception, "It dun broke"):
        return_value = yield session.call('double_fail')
        self.addDetail("return_value", return_value)

but instead I have to write:

    from testtools.content import text_content
    with ExpectedException(Exception, "It dun broke"):
        return_value = yield session.call('double_fail')
        self.addDetail("return_value", text_content(repr(return_value)))

or:

    from testtools.content import Content
    from testtools.content_type import UTF8_TEXT
    with ExpectedException(Exception, "It dun broke"):
        return_value = yield session.call('double_fail')
        self.addDetail(
            "return_value", Content(
                UTF8_TEXT, lambda: [repr(return_value)]))

It would make my life better if addDetail() did something sensible
with non-Content objects.

For example something as simple the following might be reasonable:

    def addDetail(self, name, content_object):
        if not isinstance(content_object, Content):
            content_object = text_content(repr(content_object))
        ...