Merge timeZone assignment into more clear single operation

Author: syoutsey

Foundation/NSCalendar.swift

@@ -365,13 +365,7 @@ public class NSCalendar : NSObject, NSCopying, NSSecureCoding {
     public func dateFromComponents(comps: NSDateComponents) -> NSDate? {
         var (vector, compDesc) = _convert(comps)
 
-        let oldTz = timeZone
-        let tempTz = comps.timeZone
-        if let tz = tempTz {
-            self.timeZone = tz
-        } else {
-            self.timeZone = oldTz
-        }
+        self.timeZone = comps.timeZone ?? timeZone
 
         var at: CFAbsoluteTime = 0.0
         let res: Bool = withUnsafeMutablePointer(&at) { t in