Fix Value instance type error when merging non-existent seo data

[jesseleite]

Fix Value instance type error (see #245 (comment)), when merging non-existent entry seo data.

Fixes #245.

[what-the-diff]

• The diff is changing the way that augmentedValue() works.
• It's removing a call to value(), which was previously being called on an object returned by augmentedValue().
• This change will affect all code in this project that calls $content->augmentedValue('seo'). In particular, it may cause errors if any of those places were expecting the return value from augmentedValue() to be an array instead of an object (which would have been true before).
• I'm not sure what other effects this might have because I don't know how else or where else '$content' gets used throughout the rest of the application and whether there are any assumptions about its type elsewhere in our codebase; however, since we're using PHP 7 now, we should probably just remove these types altogether so they can be inferred automatically at runtime anyway...