Reset "context" attribute on the connection class after the request finished

"context" attribute on the connection class is there to allow developers to save temporary data on the connection class for the duration of a request.

It's commonly used and accessed in places such as "parse_body" and "parse_error".

Because of the oversight, this attribute is currently not reset after the request has finished and the value of context is available across multiple requests.

This is bad and in-directly encourages bad practices like persisting some state across multiple requests.

The goal of this issue is to fix that and reset the context after the request has finished. We will also need to update the affected code, because sadly, some drivers rely on this broken and bad behavior.