Perf optimization for `url_for` called w/ Hash

Benchmarking the existing code:

```ruby
{ :only_path => options[:host].nil? }.merge!(options.symbolize_keys)) 
```

Against optimized code, that does a merge:

```ruby
options = options.symbolize_keys
options[:only_path] = options[:host].nil? unless options.key?(:only_path)
options
```

We see a statistically significant performance gain:

![](https://www.dropbox.com/s/onocpc0zfw4kjxl/Screenshot%202014-08-14%2012.45.30.png?dl=1)

Updated to not mutate incoming parameters