Ensure File.open_buffer doesn't rewrite unchanged data.

This is a backport of 14b63f6 from trunk.

lib/zip/file.rb

@@ -99,6 +99,7 @@ def initialize(path_or_io, dep_create = false, dep_buffer = false,
         end
       elsif buffer && path_or_io.size > 0
         # This zip is probably a non-empty StringIO.
+        @create = false
         read_from_stream(path_or_io)
       elsif @create
         # This zip is completely new/empty and is to be created.
@@ -411,6 +412,8 @@ def commit
 
     # Write buffer write changes to buffer and return
     def write_buffer(io = ::StringIO.new(''))
+      return unless commit_required?
+
       ::Zip::OutputStream.write_buffer(io) do |zos|
         @entry_set.each { |e| e.write_to_zip_output_stream(zos) }
         zos.comment = comment

test/file_test.rb

@@ -104,17 +104,27 @@ def test_get_output_stream
   end
 
   def test_open_buffer_with_string
-    string = File.read('test/data/rubycode.zip')
+    data = File.read('test/data/rubycode.zip', mode: 'rb')
+    string = data.dup
+
     ::Zip::File.open_buffer string do |zf|
       assert zf.entries.map(&:name).include?('zippedruby1.rb')
     end
+
+    # Ensure the buffer hasn't changed.
+    assert_equal(data, string)
   end
 
   def test_open_buffer_with_stringio
-    string_io = StringIO.new File.read('test/data/rubycode.zip')
+    data = File.read('test/data/rubycode.zip', mode: 'rb')
+    string_io = StringIO.new(data.dup)
+
     ::Zip::File.open_buffer string_io do |zf|
       assert zf.entries.map(&:name).include?('zippedruby1.rb')
     end
+
+    # Ensure the buffer hasn't changed.
+    assert_equal(data, string_io.string)
   end
 
   def test_close_buffer_with_stringio
@@ -123,6 +133,18 @@ def test_close_buffer_with_stringio
     assert_nil zf.close
   end
 
+  def test_open_buffer_without_block_write_buffer_does_nothing
+    data = File.read('test/data/rubycode.zip', mode: 'rb')
+    string_io = StringIO.new(data.dup)
+
+    zf = ::Zip::File.open_buffer(string_io)
+    assert zf.entries.map(&:name).include?('zippedruby1.rb')
+
+    # Ensure the buffer isn't changed.
+    zf.write_buffer(string_io)
+    assert_equal(data, string_io.string)
+  end
+
   def test_open_buffer_no_op_does_not_change_file
     Dir.mktmpdir do |tmp|
       test_zip = File.join(tmp, 'test.zip')