media.like function is broken - small fix

[timhuff]

At the start of the media.like function, you have the _credentials code running on an empty object (explicitly {}). This should be params instead. Was getting an OAuthParameterException. Changed line 49 of class.instagram.media.js from

params['post_data'] = this.parent._credentials({}, 'access_token');

to

params['post_data'] = this.parent._credentials(params, 'access_token');

and everything works fine now.