Reduce HTTP request payload

[KitaitiMakoto]

Using /META-INF/container.xml instead of /mimetype to guess whether given book is packaged or not reduces HTTP request payload because /META-INF/container.xml is cached.

It's better if the response is cached to JavaScript object and reused later, but this patch doesn't make anything worse.

[satorumurmur]

@KitaitiMakoto, what you say is right absolutely. Thanks. (At first, I used container.xml. But I changed it later. And now, I've forgotten the reason....)

[KitaitiMakoto]

thx!

[KitaitiMakoto]

@satorumurmur FYI

I've forgotten the reason

I thought the reason might be related to content-type HTTP header.