Simplify the keyword plugin

`.lower()` is now done in `secret_generator`
so there is no need to do it twice.

detect_secrets/plugins/keyword.py

@@ -55,7 +55,7 @@ def analyze_string(self, string, line_num, filename):
         if WHITELIST_REGEX.search(string):
             return output
 
-        for identifier in self.secret_generator(string.lower()):
+        for identifier in self.secret_generator(string):
             secret = PotentialSecret(
                 self.secret_type,
                 filename,