Fix remarks on #189 to #193
Remove _record_.py (#193)

According to https://github.com/jacquev6/PyGithub/commit/0f74e4389b3c0fa57a83083ecfbbf5c331022674#commitcomment-3919786
Separate tests for conditional requests (#193)
#193: Don't use a try-except for a usual execution flow in GithubObject.update

(and factorize assignment of headers in _storeAndUseAttributes,
as done for rawData)
#193: Add remarks
#193: Add a param to Requester.requestXxx for request headers
#193: Fix test coverage
Move method update to CompletableGithubObject
Move the DEBUG_ON_RESPONSE call to Requester.__requestEncode