feat($interpolate): use custom toString() function if present

This behavior is consistent with implementations found in other languages such as Ruby, Python,
and CoffeeScript.
http://rubymonk.com/learning/books/1-ruby-primer/chapters/5-strings/lessons/31-string-basics
https://docs.python.org/2/library/stdtypes.html#string-formatting-operations
http://coffeescriptcookbook.com/chapters/strings/interpolation

Fixes #7317
Closes #8350

BREAKING CHANGE:

When converting values to strings, interpolation now uses a custom toString() function on objects
that are not numbers, dates or arrays (custom means that the `toString()` function is not the same as
`Object.prototype.toString()`). Otherwise, interpolation uses JSON.stringify() as usual.

Should you have a custom toString() function but still want the output of JSON.stringify(), migrate
as shown in the following examples:

Before:

```html
<span>{{myObject}}</span>
```

After - use the `json` filter to stringify the object:

```html
<span>{{myObject | json}}</span>
```

Author: danielkrainas

src/ng/interpolate.js

@@ -122,7 +122,11 @@ function $InterpolateProvider() {
           value = '' + value;
           break;
         default:
-          value = toJson(value);
+          if (isFunction(value.toString) && value.toString !== Object.prototype.toString && !isArray(value)) {
+            value = value.toString();
+          } else {
+            value = toJson(value);
+          }
       }
 
       return value;

src/ngMessageFormat/messageFormatCommon.js

@@ -12,12 +12,25 @@ var noop = window['angular']['noop'],
     toJson = window['angular']['toJson'];
 
 function stringify(value) {
-  if (value == null /* null/undefined */) { return ''; }
+  if (value == null) { // null || undefined
+    return '';
+  }
   switch (typeof value) {
-    case 'string':     return value;
-    case 'number':     return '' + value;
-    default:           return toJson(value);
+    case 'string':
+      break;
+    case 'number':
+      value = '' + value;
+      break;
+    default:
+      console.log('default', value, JSON.stringify(value));
+      if (isFunction(value.toString) && value.toString !== Object.prototype.toString && !isArray(value) && !isDate(value)) {
+        value = value.toString();
+      } else {
+        value = toJson(value);
+      }
   }
+
+  return value;
 }
 
 // Convert an index into the string into line/column for use in error messages

test/ng/interpolateSpec.js

@@ -35,6 +35,22 @@ describe('$interpolate', function() {
     expect($interpolate('{{ false }}')({})).toEqual('false');
   }));
 
+  it('should use custom toString when present', inject(function($interpolate, $rootScope) {
+     var context = {
+      a : {
+        toString: function() {
+          return 'foo';
+        }
+      }
+    };
+
+    expect($interpolate('{{ a }}')(context)).toEqual('foo');
+  }));
+
+  it('should NOT use toString on array objects', inject(function($interpolate) {
+    expect($interpolate('{{a}}')({ a: [] })).toEqual('[]');
+  }));
+
 
   it('should return interpolation function', inject(function($interpolate, $rootScope) {
     var interpolateFn = $interpolate('Hello {{name}}!');