escape semicolons when serializing aggregated activities

[izhan]

I am currently running Stream-Framework with production data, and recently ran into an issue with how stream-framework serialize Aggregated Activities.

In aggregated_activity_serializer.py, we have the line check_reserved(serialized, [';', ';;']) that checks if the serialized values contain semicolons. My use case is creating notifications for when a user (actor) replies (verb) with a comment (object) to another comment (target). Due to the architecture of the current system, I currently include the body of the comment as extra_context to the activity. The comment body may contain semicolons, and a SerializationException is thrown upon serialization.

This PR simply escapes semicolons before deserialization, allowing for semicolons in activities.

I will be able to add some tests for this on Monday — just wanted to put this up first before the weekend :)

[izhan]

Closed in favor of #148 (different branch & added a test)