* Make sure collection is a list if a str is given
* Call field validation early on collections
Because we are doing work on modifying the collections value before
it is actually validated, we can validate it ourselves early to make
sure the user supplies either a string or list. Dicts are not valid.
The new validation allows us to simplify the _ensure_default_collection()
function. And since the field is now static, we no longer need to specify
a default for it, which also allows us to simplify the function. Since
the default is now removed, we can also remove the sanity/ignore.txt entry
for collectionsearch.py.
New unit tests are added (and the existing one modified) that allow us to
make sure that we throw a parser error if a user specifies something other
than a string or list for the collections value everywhere it can be specified.
* Revert removing the collection default
The default is actually used, so restore it.
* Fix unit tests in test_helpers.py affected by early collection validation
