should use python3 unicode in 'blog' demo

[youguanxinqing]

the code blog as follow:

class Application(tornado.web.Application):
    def __init__(self, db):
        ...
        settings = dict(
            blog_title=u"Tornado Blog",
            ...
        )
        super().__init__(handlers, **settings)

Since Python 3.0, the language’s str type contains Unicode characters, meaning any string created using "unicode rocks!", 'unicode rocks!', or the triple-quoted string syntax is stored as Unicode.

ref : the-string-type

[youguanxinqing]

Fix #2978

[youguanxinqing]

Merged and close issue.