Fix possible UnicodeDecodeError in warning when msgid already exists.

This happens when comparing 'Read more&hellip;' with 'Read more…'.
That should not give a warning, because they are really the same, but that would need a fix somewhere else,
in the code that gets the value of an i18n:attribute in a different way than an i18n:translate.

Added test for the proper printing of the warning.
Fix warning of msgid with different default for Python 3.
Merge branch 'master' into maurits-gettext
Minor pep8 for long lines.
Catch warnings in test_translate_existing.

Otherwise a warning is printed each time you run the tests.
Rewrite the print testing so it works in Python 3.3.