Fix #6128 and #8150

Request Type: bug

How to reproduce: it('should not double quote dates', function() {
        $httpBackend.expect('GET', '/url?date=2014-07-15T17:30:00.000Z').respond('');
        $http({url: '/url', params: {date:new Date('2014-07-15T17:30:00.000Z')}, method: 'GET'});
      })

Currently, datetimes passed into $http.get params are wrapped in double quotes and then escaped giving urls like this: /url?date=%222014-07-15T17:30:00.000Z%22

Component(s): $http

Impact: large

Complexity: small

This issue is related to: 

**Detailed Description:**



**Other Comments:**

See comments on #6128, #8150
http://stackoverflow.com/questions/20734894/difference-between-tojson-and-json-stringify

>That said, the main difference is that toJSON produces a value (a number, boolean, object, ...) that gets converted to a JSON string whereas JSON.stringify always produces a string.

It looks like special casing isDate is the correct approach - I'll submit a new commit.