Updated to match Darwin version (#439)

Author: damuellen

Foundation/NSCalendar.swift

@@ -177,7 +177,7 @@ public class Calendar: NSObject, NSCopying, NSSecureCoding {
     }
 
 
-    public class func currentCalendar() -> Calendar {
+    public class var current: Calendar {
         return CFCalendarCopyCurrent()._nsObject
     }
 

TestFoundation/TestNSCalendar.swift

@@ -80,18 +80,18 @@ class TestNSCalendar: XCTestCase {
         XCTAssertNil(calendar)
     }
 
-    func test_currentCalendarRRstability() {
+    func test_currentRRstability() {
         var AMSymbols = [String]()
         for _ in 1...10 {
-            let cal = Calendar.currentCalendar()
+            let cal = Calendar.current
             AMSymbols.append(cal.AMSymbol)
         }
 
         XCTAssertEqual(AMSymbols.count, 10, "Accessing current calendar should work over multiple callouts")
     }
 
     func test_copy() {
-        let calendar = Calendar.currentCalendar()
+        let calendar = Calendar.current
 
         //Mutate below fields and check if change is being reflected in copy.
         calendar.firstWeekday = 2