Recently I was writing about some known issue which was marked as resolved but not fixing the problem we had. The bug “W-3171561” was introduced in Spring 15 release.
Today we made a tremendous discovery.
If we change API Version of Class which contains Callout to 28, the tests are not failing! It is amazing!
I believe I have tried the same two years ago and one year ago and this workaround didn’t work then.
Probably there was some change in internal Salesforce implementation which was made this workaround possible.