fix: make transformers order deterministic

When several trasnformers are applied to the same property the order is reversed
every time plainToClass is called.

This fix make the transfomers orders consistent across multiple `plainToClass()`
calls.

Fixes #231

Author: leon19

src/metadata/MetadataStorage.ts

@@ -215,7 +215,7 @@ export class MetadataStorage {
                 }
             }
         }
-        return (metadataFromAncestorsTarget).reverse().concat((metadataFromTarget || []).reverse());
+        return (metadataFromAncestorsTarget).slice().reverse().concat((metadataFromTarget || []).slice().reverse());
     }
 
     private getAncestors(target: Function): Function[] {

test/functional/transformer-order.spec.ts

@@ -0,0 +1,28 @@
+import "reflect-metadata";
+import { plainToClass } from "../../src/index";
+import { defaultMetadataStorage } from "../../src/storage";
+import { Expose, Transform } from "../../src/decorators";
+
+describe("applying several transformations", () => {
+  beforeEach(() => defaultMetadataStorage.clear());
+  afterEach(() => defaultMetadataStorage.clear());
+
+  it("should keep the order of the applied decorators after several plainToClass() calls", () => {
+    class User {
+      @Transform(() => "Jonathan")
+      @Transform(() => "John")
+      @Expose()
+      name: string;
+    }
+
+    const firstUser = plainToClass(User, { name: "Joe" });
+    expect(firstUser.name).toEqual("John");
+
+    // Prior to this pull request [#355](https://github.com/typestack/class-transformer/pull/355)
+    // the order of the transformations was reversed after every `plainToClass()` call
+    // So after consecutive calls `User#name` would be "John" - "Jonathan" - "John" - "Jonathan"...
+    // This test ensures the last transformation is always the last one to be applied
+    const secondUser = plainToClass(User, { name: "Joe" });
+    expect(secondUser.name).toEqual("John");
+  });
+});