Add "reads after ungetc with multibyte characters in the buffer" spec to StringIO#readpartial

This one was missing in #1278, I got it to work with the
`encoding: binary` directive. The StringIO code was fine, the spec was
just incomplete.

Author: herwinw

library/stringio/readpartial_spec.rb

@@ -1,3 +1,4 @@
+# encoding: binary
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
@@ -24,6 +25,14 @@
     @string.readpartial(3).should == ", l"
   end
 
+  it "reads after ungetc with multibyte characters in the buffer" do
+    @string = StringIO.new(+"∂φ/∂x = gaîté")
+    c = @string.getc
+    @string.ungetc(c)
+    @string.readpartial(3).should == "\xE2\x88\x82"
+    @string.readpartial(3).should == "\xCF\x86/"
+  end
+
   it "reads after ungetc without data in the buffer" do
     @string = StringIO.new
     @string.write("f").should == 1