Now that readMap filters out '#' fields, when maps are nested in sets,
we exposed a related bug where a set was iterating over nested maps and
expected the '#' key to be present in those nested maps.
By skipping _all_ count fields when iterating over set keys, all is
right with the world again.
An `InstanceDiff` will include `ResourceAttrDiff` entries for the
"length" / `#` field of maps. This makes sense, since for something like
`terraform plan` it's useful to see when counts are changing.
The `DiffFieldReader` was not taking these entries into account when
reading maps out, and was therefore incorrectly returning maps that
included an extra `'#'` field, which was causing all sorts of havoc
for providers (extra tags on AWS instances, broken google compute
instance launch, possibly others).
* fixes#914 - extra tags on AWS instances
* fixes#883 - general core issue sprouted from #757
* removes the hack+TODO from #757
Sam Boyer