Fix filterMap doc and use compactMap internally (#231)

Author: M0rtyMerr

Readme.md

@@ -454,7 +454,7 @@ let resilientRequest = request.apply(requestPolicy)
 A common pattern in Rx is to filter out some values, then map the remaining ones to something else. `filterMap` allows you to do this in one step:
 
 ```swift
-// keep only odd numbers and double them
+// keep only even numbers and double them
 Observable.of(1,2,3,4,5,6)
 	.filterMap { number in
 		(number % 2 == 0) ? .ignore : .map(number * 2)

Source/RxSwift/filterMap.swift

@@ -23,12 +23,12 @@ extension ObservableType {
         - returning `.map(newValue)` will propagate newValue through the returned Observable.
      */
     public func filterMap<Result>(_ transform: @escaping (Element) throws -> FilterMap<Result>) -> Observable<Result> {
-        return flatMap { element -> Observable<Result> in
+        return compactMap { element in
             switch try transform(element) {
             case .ignore:
-                return .empty()
+                return nil
             case let .map(result):
-                return .just(result)
+                return result
             }
         }
     }