Add tests that Memory#upload and Memory#open accept kwargs

This is important when memory storage is used for tests, in which case
there might be some upload/download options passed by the application
code.

We also want to make sure there aren't any kwargs warnings being
triggered on Ruby 2.7, which could have been the case considering we
were using "*" instead of "**" (though it turns out it's not).

lib/shrine/storage/memory.rb

@@ -12,11 +12,11 @@ def initialize(store = {})
         @store = store
       end
 
-      def upload(io, id, *)
+      def upload(io, id, **)
         store[id] = io.read
       end
 
-      def open(id, *)
+      def open(id, **)
         io = StringIO.new(store.fetch(id))
         io.set_encoding(io.string.encoding) # Ruby 2.7.0 – https://bugs.ruby-lang.org/issues/16497
         io

test/storage/memory_test.rb

@@ -11,6 +11,19 @@
     assert Shrine::Storage::Linter.call(@memory)
   end
 
+  describe "#upload" do
+    it "accepts keyword arguments" do
+      @memory.upload(fakeio, "key", foo: "bar")
+    end
+  end
+
+  describe "#open" do
+    it "accepts keyword arguments" do
+      @memory.upload(fakeio, "key")
+      @memory.open("key", foo: "bar")
+    end
+  end
+
   # work around apparent bug in ruby 2.7.0
   # https://bugs.ruby-lang.org/issues/16497
   it "preserves encoding despite Encoding.default_internal set" do