ONLY dup if the hash is actuall frozen

Author: oreoshake

lib/secure_headers/headers/content_security_policy.rb

@@ -210,7 +210,7 @@ def combine_policies(original, additions)
           raise ContentSecurityPolicyConfigError.new("Attempted to override an opt-out CSP config.")
         end
 
-        original = original.dup # in case the hash is frozen
+        original = original.dup if original.frozen?
 
         # in case we would be appending to an empty directive, fill it with the default-src value
         additions.keys.each do |directive|