diff --git a/tool/downloader.rb b/tool/downloader.rb
index f212442d71..ba6ddc15c5 100644
--- a/tool/downloader.rb
+++ b/tool/downloader.rb
@@ -1,33 +1,62 @@
 require 'open-uri'
 
 class Downloader
-  def self.download(url, name, dir = nil)
-    data = URI(url).read
+  def self.mode_for(data)
+    data.start_with?("#!") ? 0755 : 0644
+  end
+
+  def self.http_options(file, since)
+    options = {}
+    if since
+      case since
+      when true
+        since = (File.mtime(file).httpdate rescue nil)
+      when Time
+        since = since.httpdate
+      end
+      if since
+        options['If-Modified-Since'] = since
+      end
+    end
+    options
+  end
+
+  # Downloader.download(url, name, [dir, [ims]])
+  #
+  # Update a file from url if newer version is available.
+  # Creates the file if the file doesn't yet exist; however, the
+  # directory where the file is being created has to exist already.
+  # If +ims+ is false, already download url regardless its last
+  # modified time.
+  #
+  # Example usage:
+  #   download 'http://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt',
+  #           'enc/unicode/data/UnicodeData.txt'
+  def self.download(url, name, dir = nil, ims = true)
     file = dir ? File.join(dir, name) : name
-    open(file, "wb", 0755) {|f| f.write(data)}
+    url = URI(url)
+    begin
+      data = url.read(http_options(file, ims))
+    rescue OpenURI::HTTPError => http_error
+      return http_error.message =~ /^304 / # 304 Not Modified
+      raise
+    end
+    mtime = nil
+    open(file, "wb", 0600) do |f|
+      f.write(data)
+      f.chmod(mode_for(data))
+      mtime = data.meta["last-modified"]
+    end
+    if mtime
+      mtime = Time.httpdate(mtime)
+      File.utime(mtime, mtime, file)
+    end
+    true
   rescue => e
     raise "failed to download #{name}\n#{e.message}: #{url}"
   end
 
-  # Update a file from url if newer version is available.
-  # Creates the file if the file doesn't yet exist; however, the
-  # directory where the file is being created has to exist already.
-  # Example usage:
-  #   download_if_modified_since 'http://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt',
-  #           'enc/unicode/data/UnicodeData.txt'
-  def self.download_if_modified_since(url, name, dir=nil, since=nil)
-    file = dir ? File.join(dir, name) : name
-    since = Date.new(1980,1,1) unless File.exist? file            # use very old date to assure file creation
-    since = File.mtime file    unless since                       # get last modification time for file
-    since = since.to_datetime   if since.respond_to? :to_datetime # convert Time/Date to DateTime
-    since = since.httpdate      if since.respond_to? :httpdate    # convert DateTime to String
-    open(url, 'If-Modified-Since' => since) do |r|
-      body = r.read
-      open(file, 'wb', 0755) { |f| f.write(body) }
-    end
-  rescue OpenURI::HTTPError => http_error
-    unless http_error.message =~ /^304 / # 304 Not Modified
-      raise "Failed to (check for) downloading #{url}: #{http_error.message}"
-    end
+  def self.download_if_modified_since(url, name, dir = nil)
+    download(url, name, dir)
   end
 end