Add standard Scala collection collect method to Observable

The current collect method on Observable has a different signature than the standard [collect|https://www.scala-lang.org/api/2.12.1/scala/collection/Iterable.html#collect[B](pf:PartialFunction[A,B]):Iterable[B]] method on Scala collections.

The suggestion here is to add the standard collections collect method on Observable, in addition to the non-standard collect already there.