bot: use optionxform()-ed setting names for existence check

This makes the existence check case insensitive, which matches the
parsing behavior of `RawConfigParser`.

The `ip` plugin has a `GeoIP_db_path` setting that will erroneously warn
users who set it using `sopel-plugins configure`, because the parser
writes out its normalized names (in this case, `geoip_db_path`) and then
the previous check using `hasattr()` wouldn't find the normalized name.

sopel/bot.py

@@ -10,6 +10,7 @@
 
 from ast import literal_eval
 from datetime import datetime
+import inspect
 import itertools
 import logging
 import re
@@ -389,8 +390,13 @@ def post_setup(self):
         """
         settings = self.settings
         for section_name, section in settings.get_defined_sections():
+            defined_options = {
+                settings.parser.optionxform(opt)
+                for opt, _ in inspect.getmembers(section)
+                if not opt.startswith('_')
+            }
             for option_name in settings.parser.options(section_name):
-                if not hasattr(section, option_name):
+                if option_name not in defined_options:
                     LOGGER.warning(
                         "Config option `%s.%s` is not defined by its section "
                         "and may not be recognized by Sopel.",