FrozenError: can't modify frozen String in http_server plugin helper

[Watson1978]

Describe the bug

When I try to add magic comment of # frozen_string_literal: true into input plugin using http_server plugin helper,
the helper raises FrozenError: can't modify frozen String.

FrozenError: can't modify frozen String: "/example/hello"
/home/watson/src/fluentd/lib/fluent/plugin_helper/http_server/server.rb:84:in `<<'
    /home/watson/src/fluentd/lib/fluent/plugin_helper/http_server/server.rb:84:in `block (2 levels) in <class:Server>'
    /home/watson/src/fluentd/test/plugin_helper/test_http_server_helper.rb:375:in `block (4 levels) in <class:HttpHelperTest>'
    /home/watson/src/fluentd/lib/fluent/plugin_helper/http_server.rb:70:in `block in http_server_create_http_server'

To Reproduce

Add magic comment of # frozen_string_literal: true into input plugin using http_server plugin helper

Expected behavior

Works well

Your Environment

- Fluentd version:
- Package version:
- Operating system:
- Kernel version:

Your Configuration

none

Your Error Log

FrozenError: can't modify frozen String: "/example/hello"
/home/watson/src/fluentd/lib/fluent/plugin_helper/http_server/server.rb:84:in `<<'
    /home/watson/src/fluentd/lib/fluent/plugin_helper/http_server/server.rb:84:in `block (2 levels) in <class:Server>'
    /home/watson/src/fluentd/test/plugin_helper/test_http_server_helper.rb:375:in `block (4 levels) in <class:HttpHelperTest>'
    /home/watson/src/fluentd/lib/fluent/plugin_helper/http_server.rb:70:in `block in http_server_create_http_server'

### Additional context

_No response_