Add spec for IO.copy_stream with partial writes

When the receiving stream doesn't accept all data written by
IO.copy_stream, the runtime should continue attempting to write
that data until what it read has been consumed.

See jruby/jruby#6078 for an example bug in JRuby.

core/io/copy_stream_spec.rb

@@ -279,4 +279,35 @@
     end
 
   end
+
+
+  describe "with a destination that does partial reads" do
+    before do
+      @from_out, @from_in = IO.pipe
+      @to_out, @to_in = IO.pipe
+    end
+
+    after do
+      [@from_out, @from_in, @to_out, @to_in].each {|io| io.close rescue nil}
+    end
+
+    it "calls #write repeatedly on the destination Object" do
+      @from_in.write "1234"
+      @from_in.close
+
+      th = Thread.new do
+        IO.copy_stream(@from_out, @to_in)
+      end
+
+      copied = ""
+      4.times do
+        copied += @to_out.read(1)
+      end
+
+      th.join
+
+      copied.should == "1234"
+    end
+
+  end
 end