net/ftp: fix FrozenError in BufferedSocket

I noticed this bug while working on something else with
RUBYOPT=-d on, existing test cases all passed with it.

Note: I use String.new because it is the local style, here,
I prefer +'' (or ''.b, for a future commit)

* lib/net/ftp.rb (BufferedSocket#read): use String.new
* test/net/ftp/test_buffered_socket.rb (test_read_nil): new test
  [ruby-core:84675] [Bug #14323]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61639 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/net/ftp.rb

@@ -1428,7 +1428,7 @@ module Net
           s = super(len, String.new, true)
           return s.empty? ? nil : s
         else
-          result = ""
+          result = String.new
           while s = super(DEFAULT_BLOCKSIZE, String.new, true)
             break if s.empty?
             result << s

test/net/ftp/test_buffered_socket.rb

@@ -33,6 +33,12 @@ class BufferedSocketTest < Test::Unit::TestCase
     assert_equal("bar", sock.gets)
   end
 
+  def test_read_nil
+    sock = create_buffered_socket("foo\nbar")
+    assert_equal("foo\nbar", sock.read)
+    assert_equal("", sock.read)
+  end
+
   private
 
   def create_buffered_socket(s)