Command Parameters do not handle NoneType

[shadetree01010100]

If a command parameter is omitted from an API request to a command, the command method will be called with omitted param(s) passed a string "None", instead of an actual NoneType.

nio-blocks/buffer#12

It looks like none of the command paramaters support this, in the buffer block use case the group param is typically a string, but can be None.

Should StringParameter handle the special case of None where allow_none = True? Is there another Parameter type that might be a better fit?