Add the before_handle_response option

[akaihola]

Fixes #544.

The problem

Currently callbacks defined using the before_record_response can modify responses, but this creates an inconsistency where e.g. unit test cases see unmodified responses when cassettes are being recorded, and modified ones when they are later played back.

The solution

The before_handle_response can be used to define callbacks instead. The response modifications now apply equally to live responses when recording cassettes as well as played back responses later:

def vcr_config():
    return {
        "before_handle_response": [my_callback],
    }

This is an alternative to the alter_live_response = True option implemented in #595.

• design discussion
  • (going on in before_record_response returns inconsistent data depending on recording or not #544)
• documentation
• unit tests
• change log