Fixes Wolox#9

ReactiveArray#last method was crashing when the array was empty.

Author: Guido Marucci Blas

ReactiveArray/ReactiveArray.swift

@@ -64,7 +64,13 @@ public final class ReactiveArray<T>: CollectionType, MutableCollectionType, Debu
     }
 
     public var last: T? {
-        return _elements[_elements.count - 1]
+        let value: T?
+        if _elements.count > 0 {
+            value = _elements[_elements.count - 1]
+        } else {
+            value = Optional.None
+        }
+        return value
     }
 
     public var debugDescription: String {

ReactiveArrayTests/ReactiveArraySpec.swift

@@ -541,6 +541,18 @@ class ReactiveArraySpec: QuickSpec {
                 expect(array.last).to(equal(4))
             }
 
+            context("when the array is empty") {
+             
+                beforeEach {
+                    array = ReactiveArray<Int>()
+                }
+                
+                it("returns .None") {
+                    expect(array.last).to(beNil())
+                }
+                
+            }
+            
         }
 
     }