Add developer warning when replacing SC.ArrayController content

[mauritslamers]

A well known source for memory leaks in SproutCore applications is replacing a SC.RecordArray instance with a new one as content in an array controller without destroying the original instance first. This causes a memory leak because the store will hold on to a reference of the record array instance in order to update it whenever new information is available.

I propose to add the following to SC.ArrayController in order to give a developer warning when someone tries to replace a record array instance which is not destroyed. As SC.ArrayController is in the core_foundation framework, it uses a simple check on properties that need to exists on a record aray instead of using SC.instanceOf(SC.RecordArray).

  set: function (key, value) {
    if (key === "content") {
      var c = this.get('content');
      if (c.store || c.query) {
        if (!c.get('isDestroyed')) {
          SC.warn("Developer Warning: You are replacing the content of an array controller with a new record array without destroying the old one. This causes memory leaks!");
        }
      }
    }
    sc_super();
  },

Edit: grammar

[nicolasbadia]

I really like the idea, I will give it a try.
I suggest that we wrap this in //@if (debug) statements.

[mauritslamers]

It should most certainly go in an //@if debug block. On top of SC.ArrayController there is already one with toString. I intended it to go below (that is where I wrote it). I am just not sure whether this should go to master now, or to 2-0-unstable.