refactor: change allKeys to a computed propriety

Sources/KeychainSwift.swift

@@ -237,7 +237,7 @@ open class KeychainSwift {
      
      - returns: An string array with all keys on keychain
      */
-    open func allKeys() -> [String] {
+    public var allKeys: [String] {
         var query: [String: Any] = [
             KeychainSwiftConstants.klass : kSecClassGenericPassword,
             KeychainSwiftConstants.returnData : true,

Tests/KeychainSwiftTests/AllKeysTests.swift

@@ -31,10 +31,10 @@ class AllKeysTests: XCTestCase {
         self.obj!.set("\(enumerator.offset)", forKey: enumerator.element)
     }
     
-    XCTAssertEqual(["one", "two"], obj.allKeys())
+    XCTAssertEqual(["one", "two"], obj.allKeys)
     
     obj.clear()
-    XCTAssertEqual(obj.allKeys(), [])
+    XCTAssertEqual(obj.allKeys, [])
     
   }