Simplify and improve performance of the keyword plugin

As per @domanchi comments for the PR #83, doing `lower()` once
is enough, and should also be faster.

detect_secrets/plugins/keyword.py

@@ -32,7 +32,9 @@
 
 
 BLACKLIST = (
-    'PASS =',
+    # NOTE all values here should be lowercase,
+    # otherwise _secret_generator can fail to match them
+    'pass =',
     'password',
     'passwd',
     'pwd',
@@ -66,7 +68,10 @@ def analyze_string(self, string, line_num, filename):
 
         return output
 
-    def secret_generator(self, string):
+    def _secret_generator(self, lowercase_string):
         for line in BLACKLIST:
-            if line.lower() in string.lower():
+            if line in lowercase_string:
                 yield line
+
+    def secret_generator(self, string):
+        return self._secret_generator(string.lower())