use @+ operator instead of #dup to get unfrozen string

lib/zip/crypto/decrypted_io.rb

@@ -25,7 +25,7 @@ module Zip
     end
 
     def buffer
-      @buffer ||= ''.dup
+      @buffer ||= +''
     end
 
     def input_finished?

lib/zip/entry.rb

@@ -615,7 +615,7 @@ module Zip
         get_input_stream do |is|
           bytes_written = 0
           warned = false
-          buf = ''.dup
+          buf = +''
           while (buf = is.sysread(::Zip::Decompressor::CHUNK_SIZE, buf))
             os << buf
             bytes_written += buf.bytesize

lib/zip/extra_field.rb

@@ -26,7 +26,7 @@ module Zip
     end
 
     def create_unknown_item
-      s = ''.dup
+      s = +''
       class << s
         alias_method :to_c_dir_bin, :to_s
         alias_method :to_local_bin, :to_s

lib/zip/file.rb

@@ -402,7 +402,7 @@ module Zip
     # Creates a directory
     def mkdir(entryName, permissionInt = 0o755)
       raise Errno::EEXIST, "File exists - #{entryName}" if find_entry(entryName)
-      entryName = entryName.dup.to_s
+      entryName = +entryName.to_s
       entryName << '/' unless entryName.end_with?('/')
       @entry_set << ::Zip::StreamableDirectory.new(@name, entryName, nil, permissionInt)
     end

lib/zip/inflater.rb

@@ -3,7 +3,7 @@ module Zip
     def initialize(*args)
       super
 
-      @buffer = ''.dup
+      @buffer = +''
       @zlib_inflater = ::Zlib::Inflate.new(-Zlib::MAX_WBITS)
     end