Make decode_responses=True the default

[Grokzen]

In 3.0.0 this flag will be set to default True as in 3.0.0 the python 2.7 backwards compatibility will be dropped and that flag is mostly for python 3 and not much else. This will be a breaking change as it changes the default behavior. A possible good feature could be that if the user has explicitly set the value to False when running with 3.0.0 then it should raise deprecation warning or some other error/warning message up to the user that the feature has changed. Also document this in the upgrading instructions document.