Double the speed for Utils.build_nested_query for handling large arrays

The branch to take the prefix will be taken at most one time when
inside the loop, and this code is only called if the array is non-empty,
meaning it is called at least one time.  So it's safe to move it
outside the loop.

While here, change the probably wrong regexp use with $ to a
String#end_with? call.

I tried using the buffer-based approach used for build_multipart
in build_nested_query, but it did not provide a measurable speedup,
and made the method measurably slower for some inputs.

lib/rack/test/utils.rb

@@ -14,8 +14,8 @@ def build_nested_query(value, prefix = nil)
           if value.empty?
             "#{prefix}[]="
           else
+            prefix += "[]" unless unescape(prefix).end_with?('[]')
             value.map do |v|
-              prefix = "#{prefix}[]" unless unescape(prefix) =~ /\[\]$/
               build_nested_query(v, prefix.to_s)
             end.join('&')
           end