Fix issue with model attribute matching metasearch method name

Not sure if you're still accepting PRs fixing 0.6 only issues. This is a fix for the case when an attribute on a model matches a method on the metasearch search object. In my case, it was a string attribute called `size`. I was expecting it to create a standard string filter, but got something like this instead:

![screen shot 2014-12-16 at 3 26 03 pm](https://cloud.githubusercontent.com/assets/495973/5464059/05391e0a-8538-11e4-81d9-a2efaaedef9a.png)

(757 is the actual count of records of this model).

After this fix, it looks like you'd expect:

![screen shot 2014-12-16 at 3 40 08 pm](https://cloud.githubusercontent.com/assets/495973/5464194/e01bd390-8539-11e4-9ac9-0afe23fba6a7.png)

I checked metasearch's docs to see if there were a better method for determining what methods are actually search methods but didn't see anything, so this seemed like the simplest fix.