Fix undefined method 'to_ber' for nil:NilClass. Convert unless nil.

Made modify_ops a method to allow for unit testing of it.

Added unit tests for add, delete and replace.

test/test_ldap_connection.rb

@@ -0,0 +1,24 @@
+require 'common'
+
+class TestLDAP < Test::Unit::TestCase
+    def test_modify_ops_delete
+        args = {:operations=>[[:delete, "mail"]]}
+        result = Net::LDAP::Connection.modify_ops(args)
+        expected = ["0\r\n\x01\x010\b\x04\x04mail1\x00"]
+        assert_equal(expected, result)
+    end
+
+    def test_modify_ops_add
+        args = {:operations=>[[:add, "mail", "testuser@example.com"]]}
+        result = Net::LDAP::Connection.modify_ops(args)
+        expected = ["0#\n\x01\x000\x1E\x04\x04mail1\x16\x04\x14testuser@example.com"]
+        assert_equal(expected, result)
+    end
+
+    def test_modify_ops_replace
+        args = {:operations=>[[:replace, "mail", "testuser@example.com"]]}
+        result = Net::LDAP::Connection.modify_ops(args)
+        expected = ["0#\n\x01\x020\x1E\x04\x04mail1\x16\x04\x14testuser@example.com"]
+        assert_equal(expected, result)
+    end
+end