Fix a problem that parse exception message can't be generated for invalid encoding XML (#123)

## Why?

If the XML tag contains Unicode characters and an error is occurred for
the tag, an incompatible encoding error is raised. Because our parse
exception message parts have an UTF-8 part (that includes the target tag
information) and an ASCII-8BIT part (that includes error context input).

Fix GH-29

Reported by DuKewu. Thanks!!!

Author: naitoh

lib/rexml/parseexception.rb

@@ -29,6 +29,7 @@ def to_s
         err << "\nLine: #{line}\n"
         err << "Position: #{position}\n"
         err << "Last 80 unconsumed characters:\n"
+        err.force_encoding("ASCII-8BIT")
         err << @source.buffer[0..80].force_encoding("ASCII-8BIT").gsub(/\n/, ' ')
       end
 

test/parse/test_element.rb

@@ -47,6 +47,19 @@ def test_empty_namespace_attribute_name
         DETAIL
       end
 
+      def test_empty_namespace_attribute_name_with_utf8_character
+        exception = assert_raise(REXML::ParseException) do
+          parse("<x :\xE2\x80\x8B>") # U+200B ZERO WIDTH SPACE
+        end
+        assert_equal(<<-DETAIL.chomp.force_encoding("ASCII-8BIT"), exception.to_s)
+Invalid attribute name: <:\xE2\x80\x8B>
+Line: 1
+Position: 8
+Last 80 unconsumed characters:
+:\xE2\x80\x8B>
+        DETAIL
+      end
+
       def test_garbage_less_than_before_root_element_at_line_start
         exception = assert_raise(REXML::ParseException) do
           parse("<\n<x/>")