Ignore invalid characters in strings.

Multiple ADs return invalid characters and we should ignore them.

This is the same as the pull request 44 on the original project.

Author: Timur Alperovich

lib/net/ber/core_ext/string.rb

@@ -29,7 +29,8 @@ def raw_utf8_encoded
       # Strings should be UTF-8 encoded according to LDAP.
       # However, the BER code is not necessarily valid UTF-8
       begin
-        self.encode('UTF-8').force_encoding('ASCII-8BIT')
+        self.encode('UTF-8', :invalid => :replace, :undef => :replace,
+                    :replace => '').force_encoding('ASCII-8BIT')
       rescue Encoding::UndefinedConversionError
         self
       end