Fix regression from simple_stub

Since introducing simple_stub, this has been broken:

``` ruby
    dbl = double("foo" => 1)
    allow(dbl).to receive("foo").and_return(2)
```

The `receive` matcher converts `"foo"` to `:foo`, but simple stub was not converting `"foo"`, so we wound up with two method doubles (one for the string, one for the symbol) where we should have had one and things blew up when trying to reset the method doubles.