Remove unneeded flatten

lib/secure_headers/headers/policy_management.rb

@@ -109,7 +109,7 @@ module SecureHeaders
       DIRECTIVES_2_0 + DIRECTIVES_DRAFT
     ).freeze
 
-    ALL_DIRECTIVES = [DIRECTIVES_1_0 + DIRECTIVES_2_0 + DIRECTIVES_3_0 + DIRECTIVES_DRAFT].flatten.uniq.sort
+    ALL_DIRECTIVES = (DIRECTIVES_1_0 + DIRECTIVES_2_0 + DIRECTIVES_3_0 + DIRECTIVES_DRAFT).uniq.sort
 
     # Think of default-src and report-uri as the beginning and end respectively,
     # everything else is in between.