fix(interpolate): Interpolation should use toString instead of json.stringify

Closes #60

Author: pavelgj

lib/interpolate.dart

@@ -9,7 +9,7 @@ num endSymbolLength = endSymbol.length;
 class Interpolate {
   Parser $parse;
   ExceptionHandler $exceptionHandler;
-  
+
   Interpolate(Parser this.$parse, ExceptionHandler this.$exceptionHandler);
 
   ParsedFn call(String text, [bool mustHaveExpression = false]) {
@@ -54,7 +54,7 @@ class Interpolate {
               if (chunk == null) {
                 chunk = '';
               } else if (!(chunk is String)) {
-                chunk = toJson(chunk);
+                chunk = '$chunk';
               }
             }
             concat.add(chunk);

test/interpolate_spec.dart

@@ -1,5 +1,9 @@
 import "_specs.dart";
 
+class ToStringableObject {
+  toString() => 'World';
+}
+
 main() {
   describe('\$interpolate', () {
 
@@ -48,6 +52,11 @@ main() {
       expect($interpolate("Hello {{'World' + foo()}}")($rootScope)).toEqual('Hello World');
     }));
 
+    it('should use toString to conver objects to string', inject((Interpolate $interpolate, Scope $rootScope) {
+      $rootScope.obj = new ToStringableObject();
+      expect($interpolate("Hello, {{obj}}!")($rootScope)).toEqual('Hello, World!');
+    }));
+
 
     describe('parseBindings', () {
       it('should Parse Text With No Bindings', inject((Interpolate $interpolate) {