IndexingTestDb: support collection.indexSort

Sort the mock index if the indexSort method is provided by the collection.
IndexCollection: reset the temporary comparator used by fetchModels

Previously for collections with no provided comparator,
the first call to fetch() that got a non-empty result fixed the comparator
for the posterity, capturing the sequence of ids retrieved during the first fetch
as the sorting order. All subsequent additions to the index got incorrectly sorted.
Test indexSort on IndexCollection
Merge pull request #14 from Everyplay/bug/index-sort

Fix sorting bugs in IndexCollection and the mock database