fix indexing "watch" on firefox with memory_store

A surprisingly tricky bug. When trying to index a set of documents that includes the word "watch" on Firefox with the memory store I was getting an error with `this.data[key].insert(value)` saying that .insert() wasn't defined.

As it turns out, the check to see if the key `"watch"` was present in `this.data` was giving a false positive. Switching to `Object#hasOwnProperty` seems to fix it.