in_http: replace WEBrick::HTTPUtils.parse_query with URI (#4900)

**Which issue(s) this PR fixes**: 
* Partially Fixes #4648

**What this PR does / why we need it**: 
Replace `WEBrick::HTTPUtils.parse_query` with `URI.decode_www_form` in
`in_http`.

WEBrick is no longer recommended for production use.

`WEBrick::HTTPUtils` is just a util, so it might be harmless.
However, it would be happy to reduce dependence if possible.

This changes some specifications unintentionally.
At least, this makes us unable to use `;` delimitter.
We need to consider whether this breaking change is acceptable or not.

**Docs Changes**:
Not needed.

**Release Note**: 
The same as the title.

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

lib/fluent/plugin/in_http.rb

@@ -531,12 +531,12 @@ def on_message_complete
         @env['REMOTE_ADDR'] = @remote_addr if @remote_addr
 
         uri = URI.parse(@parser.request_url)
-        params = WEBrick::HTTPUtils.parse_query(uri.query)
+        params = parse_query(uri.query)
 
         if @format_name != 'default'
           params[EVENT_RECORD_PARAMETER] = @body
         elsif /^application\/x-www-form-urlencoded/.match?(@content_type)
-          params.update WEBrick::HTTPUtils.parse_query(@body)
+          params.update parse_query(@body)
         elsif @content_type =~ /^multipart\/form-data; boundary=(.+)/
           boundary = WEBrick::HTTPUtils.dequote($1)
           params.update WEBrick::HTTPUtils.parse_form_data(@body, boundary)
@@ -553,7 +553,7 @@ def on_message_complete
 
         if (@add_query_params)
 
-          query_params = WEBrick::HTTPUtils.parse_query(uri.query)
+          query_params = parse_query(uri.query)
 
           query_params.each_pair {|k,v|
             params["QUERY_#{k.tr('-','_').upcase}"] = v
@@ -643,6 +643,10 @@ def include_cors_allow_origin
 
         !r.nil?
       end
+
+      def parse_query(query)
+        query.nil? ? {} : Hash[URI.decode_www_form(query, Encoding::ASCII_8BIT)]
+      end
     end
   end
 end