Fix Cancellable for ver 3.x API compatibility.

Author: inamiy

SwiftTask/Cancellable.swift

@@ -12,7 +12,13 @@ public protocol Cancellable
 {
     typealias Error
 
-    func cancel(error: Error) -> Bool
+    //
+    // NOTE:
+    // Single `func cancel(error: Error) -> Bool` is preferred (as first implemented in 8a22ed5),
+    // but two overloaded methods are required for SwiftTask ver 3.x API compatibility.
+    //
+    func cancel() -> Bool
+    func cancel(#error: Error) -> Bool
 }
 
 public class Canceller: Cancellable
@@ -24,7 +30,12 @@ public class Canceller: Cancellable
         self.cancelHandler = cancelHandler
     }
 
-    public func cancel(error: Void) -> Bool
+    public func cancel() -> Bool
+    {
+        return self.cancel(error: ())
+    }
+    
+    public func cancel(#error: Void) -> Bool
     {
         if let cancelHandler = self.cancelHandler {
             self.cancelHandler = nil

SwiftTask/SwiftTask.swift

@@ -564,10 +564,10 @@ public class Task<Progress, Value, Error>: Cancellable, Printable
     //
     public func cancel() -> Bool
     {
-        return self.cancel(nil)
+        return self.cancel(error: nil)
     }
 
-    public func cancel(error: Error?) -> Bool
+    public func cancel(#error: Error?) -> Bool
     {
         return self._cancel(error: error)
     }

SwiftTaskTests/SwiftTaskTests.swift

@@ -598,7 +598,7 @@ class SwiftTaskTests: _TestCase
         // cancel at time between 1st & 2nd delay (t=0.3)
         Async.main(after: 0.3) {
 
-            task.cancel("I get bored.")
+            task.cancel(error: "I get bored.")
 
             XCTAssertEqual(task.state, TaskState.Cancelled)
 
@@ -635,7 +635,7 @@ class SwiftTaskTests: _TestCase
         // cancel task3 at time between task1 fulfilled & before task2 completed (t=0.6)
         Async.main(after: 0.6) {
 
-            task3.cancel("I get bored.")
+            task3.cancel(error: "I get bored.")
 
             XCTAssertEqual(task3.state, TaskState.Cancelled)
 
@@ -1175,7 +1175,7 @@ class SwiftTaskTests: _TestCase
 
         // cancel before fulfilled
         Async.main(after: 0.01) {
-            groupedTask.cancel("Cancel")
+            groupedTask.cancel(error: "Cancel")
             return
         }
 
@@ -1390,7 +1390,7 @@ class SwiftTaskTests: _TestCase
 
         // cancel before fulfilled
         self.perform {
-            groupedTask.cancel("Cancel")
+            groupedTask.cancel(error: "Cancel")
             return
         }