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
```

lib/imgix/path.rb

@@ -21,10 +21,11 @@ def to_url(opts = {})
       prev_options = @options.dup
       @options.merge!(opts)
 
-      url = @prefix + path_and_params
+      current_path_and_params = path_and_params
+      url = @prefix + current_path_and_params
 
       if @secure_url_token
-        url += (has_query? ? "&" : "?") + "s=#{signature}"
+        url += (has_query? ? "&" : "?") + "s=#{signature(current_path_and_params)}"
       end
 
       @options = prev_options
@@ -129,8 +130,8 @@ def method_missing(method, *args, &block)
 
     private
 
-    def signature
-      Digest::MD5.hexdigest(@secure_url_token + path_and_params)
+    def signature(current_path_and_params)
+      Digest::MD5.hexdigest(@secure_url_token + current_path_and_params)
     end
 
     def path_and_params