[seiga] Support image metadata and fix cookie check.

[kurumigi]

In niconico seiga, image page HTMLs are including metadata. I want to use metadata, so implemented. And fixed cookie check process.

[mikf]

Thanks for your contribution, but next time please don't "fix" a failing test by modifying it to fit your changes. (6ac568c)

_check_cookies() wants a list or tuple of cookie names, and you changed it to a string instead.

• ("user_session") is the string user_session in parenthesis
• ("user_session",) is a tuple with 1 element, the string user_session

The first would check for cookies named u, s, e, r and so on,
while the last would only check for one cookie named user_session