perf: avoid calculating path_and_params twice
perf: avoid calculating path_and_params twice

Benchmark output:

```
❯ bundle exec ruby script/bench_to_url.rb
Warming up --------------------------------------
  Imgix::Path#to_url     2.731k i/100ms
Calculating -------------------------------------
  Imgix::Path#to_url     27.278k (± 0.7%) i/s -    136.550k in   5.006213s
```

imgix/imgix-rb