Change summary lang key default value to "the page"

[nahuelhds]

As stated by @edsu at #74

All that being said, maybe it's best to stick with summary since that is what Readability calls it, and its what is in the model. But I think the default string value should be "the page"?

[edsu]

Sorry, my mistake. I think leaving the key as summary works since it matches the data model. I thought that there might be a default value, but it appears that there is not?

[nahuelhds]

There is no default value because every code I've added to the project was intended for adding features without affecting the current default behavior that the engine already has.

I'm thinking that we could set a default value if we add a new prop for enable/disable this feature.