Investigate weird serialization logic

[tillkruss]

My best guess is that this is never called. Thoughts @naxvog?

In 3.0 we should re-think the serialization.

[naxvog]

Just to summarize our slack conversation:

• The original change was made years ago
• The last condition of maybe_serialize serializes an already serialized string (nonsense)
• All non strings should be serialized

We agreed on taking no further action at the moment in 2.x but will refactor the method in a future 3.0 release in order to maintain compatibility.

[tillkruss]

@naxvog Thoughts on the latest push?

[tillkruss]

Alright:

• We're now flush the cache right after upgrading to 2.0.16, to flush weird double serialized data
• All data going into the object cache is serialized (strings, booleans, etc.)
• All data going out of the cache is unserialized once

@naxvog: I haven't tested any of this, but I'm happy with the overall approch