Silence encoding warnings in specs

Setting `Encoding.default_external` prints a warning when running specs:

   warning: setting Encoding.default_external

Wrapping this with a `silence_warnings` block, silences the warnings.

spec/actions/inject_into_file_spec.rb

@@ -159,11 +159,15 @@ def file
       encoding_original = Encoding.default_external
 
       begin
-        Encoding.default_external = Encoding.find("UTF-8")
+        silence_warnings do
+          Encoding.default_external = Encoding.find("UTF-8")
+        end
         invoke! "doc/README.zh", "\n中文", after: "__start__"
         expect(File.read(File.join(destination_root, "doc/README.zh"))).to eq("__start__\n中文\n说明\n__end__\n")
       ensure
-        Encoding.default_external = encoding_original
+        silence_warnings do
+          Encoding.default_external = encoding_original
+        end
       end
     end
   end