Handle the case where the hash is frozen and we're adding a directive

Previously, this would cause an exception:

RuntimeError:
  can't modify frozen Hash
 ./lib/secure_headers/headers/content_security_policy.rb:216:in `block in combine_policies'
 ./lib/secure_headers/headers/content_security_policy.rb:214:in `each'
 ./lib/secure_headers/headers/content_security_policy.rb:214:in `combine_policies'
 ./spec/lib/secure_headers/headers/content_security_policy_spec.rb:127:in `block (3 levels) in <module:SecureHeaders>'

Author: oreoshake

lib/secure_headers/headers/content_security_policy.rb

@@ -210,6 +210,8 @@ 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
+
         # in case we would be appending to an empty directive, fill it with the default-src value
         additions.keys.each do |directive|
           unless original[directive] || !source_list?(directive)

spec/lib/secure_headers/headers/content_security_policy_spec.rb

@@ -117,6 +117,17 @@ module SecureHeaders
         expect(csp.value).to eq("default-src https:; script-src https: anothercdn.com")
       end
 
+      it "combines directives where the original value is nil and the hash is frozen" do
+        Configuration.default do |config|
+          config.csp = {
+            default_src: %w('self'),
+            report_only: false
+          }.freeze
+        end
+        combined_config = CSP.combine_policies(Configuration.get.csp, report_uri: %w(https://report-uri.io/asdf))
+        expect(combined_config[:report_uri]).to_not be_nil
+      end
+
       it "overrides the report_only flag" do
         Configuration.default do |config|
           config.csp = {