Merge pull request #431 from taichi-ishitani/fix_frozen_error

Fix frozen error caused by frozen string literal

.travis.yml

@@ -5,6 +5,7 @@ rvm:
   - 2.4
   - 2.5
   - 2.6
+  - 2.7
   - ruby-head
 matrix:
   fast_finish: true

lib/zip/crypto/decrypted_io.rb

@@ -7,7 +7,7 @@ def initialize(io, decrypter)
       @decrypter = decrypter
     end
 
-    def read(length = nil, outbuf = '')
+    def read(length = nil, outbuf = +'')
       return ((length.nil? || length.zero?) ? "" : nil) if eof
 
       while length.nil? || (buffer.bytesize < length)
@@ -25,7 +25,7 @@ def eof
     end
 
     def buffer
-      @buffer ||= ''.dup
+      @buffer ||= +''
     end
 
     def input_finished?

lib/zip/entry.rb

@@ -615,7 +615,7 @@ def create_file(dest_path, _continue_on_exists_proc = proc { Zip.continue_on_exi
         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 @@ def extra_field_type_unknown(binstr, len, i)
     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/inflater.rb

@@ -3,7 +3,7 @@ class Inflater < Decompressor #:nodoc:all
     def initialize(*args)
       super
 
-      @buffer = ''.dup
+      @buffer = +''
       @zlib_inflater = ::Zlib::Inflate.new(-Zlib::MAX_WBITS)
     end