Potential fix.

lib/webrick/httprequest.rb

@@ -318,7 +318,7 @@ def content_type
     def [](header_name)
       if @header
         value = @header[header_name.downcase]
-        value.empty? ? nil : value.join(", ")
+        value.empty? ? nil : value.join
       end
     end
 
@@ -329,7 +329,7 @@ def each
       if @header
         @header.each{|k, v|
           value = @header[k]
-          yield(k, value.empty? ? nil : value.join(", "))
+          yield(k, value.empty? ? nil : value.join)
         }
       end
     end

lib/webrick/httputils.rb

@@ -152,6 +152,22 @@ def mime_type(filename, mime_tab)
     # Parses an HTTP header +raw+ into a hash of header fields with an Array
     # of values.
 
+    class SplitHeader < Array
+      def join(separator = ", ")
+        super
+      end
+    end
+
+    class CookieHeader < Array
+      def join(separator = "; ")
+        super
+      end
+    end
+
+    HEADER_CLASSES = Hash.new(SplitHeader).update({
+      "cookie" => CookieHeader,
+    })
+
     def parse_header(raw)
       header = Hash.new([].freeze)
       field = nil
@@ -160,7 +176,7 @@ def parse_header(raw)
         when /^([A-Za-z0-9!\#$%&'*+\-.^_`|~]+):(.*?)\z/om
           field, value = $1, $2.strip
           field.downcase!
-          header[field] = [] unless header.has_key?(field)
+          header[field] = HEADER_CLASSES[field].new unless header.has_key?(field)
           header[field] << value
         when /^\s+(.*?)/om
           value = line.strip