Switch newly created StringIOs to binmode.

StringIO objects created within File.open_buffer were not being switched into
binmode, but those passed in were. Fix this inconsistency and add a test.

lib/zip/file.rb

@@ -134,12 +134,10 @@ module Zip
           raise "Zip::File.open_buffer expects a String or IO-like argument (responds to #{IO_METHODS.join(', ')}). Found: #{io.class}"
         end
 
-        if io.is_a?(::String)
-          io = ::StringIO.new(io)
-        elsif io.respond_to?(:binmode)
+        io = ::StringIO.new(io) if io.is_a?(::String)
+
         # https://github.com/rubyzip/rubyzip/issues/119
-          io.binmode
-        end
+        io.binmode if io.respond_to?(:binmode)
 
         zf = ::Zip::File.new(io, true, true, options)
         return zf unless block_given?

test/file_test.rb

@@ -97,6 +97,13 @@ class ZipFileTest < MiniTest::Test
     end
   end
 
+  def test_open_buffer_with_string
+    string = File.read('test/data/rubycode.zip')
+    ::Zip::File.open_buffer string do |zf|
+      assert zf.entries.map { |e| e.name }.include?('zippedruby1.rb')
+    end
+  end
+
   def test_open_buffer_with_stringio
     string_io = StringIO.new File.read('test/data/rubycode.zip')
     ::Zip::File.open_buffer string_io do |zf|