fix issue #255 - cancel a pending operation only if there is an operation pending
add test spec for fix of #255

This test is a bit constructed, because I do not know exact
source of core:cancel event, which triggers call of viewModel.cancel
add test spec for fix of issue #255
make cancel operation spec clearer
Merge pull request #290 from klorenz/master

fix issue #255 - cancel a pending operation only if there is an operatio...