ReadProperty::read_multiple should populate trends

[gnmerritt]

Ok, another small follow-on to #229 - looks like the point._trend function gets called in the ReadPropertyMultiple version and that's how polling actually populates point.history. So I've copied that logic over to the single read version.

[coveralls]

Coverage decreased (-1.5%) to 45.877% when pulling bff1746 on gnmerritt:readproperty-read-multiple-trend into de367f0 on ChristianTremblay:develop.

[ChristianTremblay]

Thank you