lib/webrick: explicitly convert header values to a string

The values of @header are expected to be all strings;
WEBrick::HTTPResponse::[]=(key, val) explicitly converts the second
argument to a string and assigns it to @header hash.
However, there were some points in WEBrick internal code that assigns
non-String to @header.  This change fixes the issues.

The values are checked by `header_value =~ /\r\n/` in check_header.
The type confusion caused conflict with removal of `Object#=~`
[Feature #15231].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65984 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/webrick/httpservlet/filehandler.rb

@@ -55,7 +55,7 @@ module WEBrick
         else
           mtype = HTTPUtils::mime_type(@local_path, @config[:MimeTypes])
           res['content-type'] = mtype
-          res['content-length'] = st.size
+          res['content-length'] = st.size.to_s
           res['last-modified'] = mtime.httpdate
           res.body = File.open(@local_path, "rb")
         end
@@ -144,7 +144,7 @@ module WEBrick
             raise HTTPStatus::RequestRangeNotSatisfiable if first < 0
             res['content-type'] = mtype
             res['content-range'] = "bytes #{first}-#{last}/#{filesize}"
-            res['content-length'] = last - first + 1
+            res['content-length'] = (last - first + 1).to_s
             res.body = io.dup
           else
             raise HTTPStatus::BadRequest