Allow changing selectors. Closes #169.

There's no reason to prevent changing selectors at any time.
Assert should not be used for runtime checks.

Author: TkTech

flask_babel/__init__.py

@@ -120,8 +120,6 @@ def localeselector(self, f):
 
         This has to return the locale as string (eg: ``'de_AT'``, ``'en_US'``)
         """
-        assert self.locale_selector_func is None, \
-            'a localeselector function is already registered'
         self.locale_selector_func = f
         return f
 
@@ -133,8 +131,6 @@ def timezoneselector(self, f):
 
         This has to return the timezone as string (eg: ``'Europe/Vienna'``)
         """
-        assert self.timezone_selector_func is None, \
-            'a timezoneselector function is already registered'
         self.timezone_selector_func = f
         return f