Fix Scan/Reduce/Collect Factory Ambiguity

This puts the seed factory on `collect` and removes it from `scan` and `reduce` due to ambiguity.
See https://github.com/ReactiveX/RxJava/pull/1883 and https://github.com/ReactiveX/RxJava/issues/1881
Merge pull request #1884 from benjchristensen/mutable-collect-scan-reduce

Fix Scan/Reduce/Collect Factory Ambiguity