Add more specs for write transcoding

Author: eregon

spec/ruby/core/io/shared/write.rb

@@ -120,6 +120,15 @@
 
     result.bytes.should == expected
   end
+
+  it "transcodes the given string when the external encoding is set and the string encoding is BINARY" do
+    str = "été".b
+
+    File.open(@transcode_filename, "w", external_encoding: Encoding::UTF_16BE) do |file|
+      file.external_encoding.should == Encoding::UTF_16BE
+      -> { file.send(@method, str) }.should raise_error(Encoding::UndefinedConversionError)
+    end
+  end
 end
 
 describe :io_write_no_transcode, shared: true do

spec/ruby/library/stringio/shared/write.rb

@@ -90,16 +90,24 @@
   end
 
   it "transcodes the given string when the external encoding is set and neither is BINARY" do
-    io = StringIO.new.set_encoding(Encoding::UTF_16BE)
     utf8_str = "hello"
+    io = StringIO.new.set_encoding(Encoding::UTF_16BE)
+    io.external_encoding.should == Encoding::UTF_16BE
 
     io.send(@method, utf8_str)
 
-    result = io.string
     expected = [0, 104, 0, 101, 0, 108, 0, 108, 0, 111] # UTF-16BE bytes for "hello"
+    io.string.bytes.should == expected
+  end
 
+  it "does not transcode the given string when the external encoding is set and the string encoding is BINARY" do
+    str = "été".b
+    io = StringIO.new.set_encoding(Encoding::UTF_16BE)
     io.external_encoding.should == Encoding::UTF_16BE
-    result.bytes.should == expected
+
+    io.send(@method, str)
+
+    io.string.bytes.should == str.bytes
   end
 end