Choose clears its value unconditionally after emitting it

[metafacture]

Choose clears its value unconditionally after emitting it

[cboehme]

choose should be resetable

[cboehme]

See issue #210 which requests choose to be resetable. Once this is implemented, it is no longer necessary to clear the value of choose after emitting it, as then this can be controlled using the reset attribute. The default value of the reset attribute should be true so that the behaviour of choose does not differ to much from the current behaviour. What will differ, though, is that the value will be reset even if an if-condition in the choose collector is not met. This is the same behaviour shown be all other collectors.