Several incr/decr functions do not convert the value to bytes

[bmerry]

The implementations of decr, hincrby and hincrbyfloat store the int/float value internally, instead of the stringified version. This can lead to errors e.g.

>>> f = fakeredis.FakeStrictRedis()
>>> f.decr('key', 1)
-1
>>> f.append('key', 'x')
TypeError: unsupported operand type(s) for +=: 'int' and 'str'

I have a fix in a branch I am preparing for PR.

[bmerry]

This is a duplicate of #90, so I'm closing it.