WRITE_MULTIPLE_COILS writes one coil past range

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What steps will reproduce the problem?

This test currently fails:

    def testWriteMultipleCoilsOffByOne(self):
        """Test that correct coils are written at range edges by writing range within range"""
        result = self.master.execute(1, modbus_tk.defines.WRITE_MULTIPLE_COILS, 0, output_value=[1]*20)
        self.assertEqual((0, 20), result)
        result = self.master.execute(1, modbus_tk.defines.WRITE_MULTIPLE_COILS, 5, output_value=[0]*10)
        self.assertEqual((5, 10), result)
        self.assertEqual(tuple([1]*5+[0]*10+[1]*5), self.slave1.get_values("c0-100", 0, 20))

What is the expected output? What do you see instead?
       See above

What version of the product are you using? On what operating system?
      0.4.3

Please provide any additional information below.
     I have the fix and associated test case.  I will push it once I figure out how mercurial works.

Original issue reported on code.google.com by barck...@gmail.com on 27 Feb 2015 at 3:27