Fix code style and add tests

Library/Homebrew/cask/artifact/abstract_artifact.rb

@@ -99,11 +99,11 @@ module Cask
         description = key ? "#{stanza} #{key.inspect}" : stanza.to_s
 
         # backward-compatible string value
-        if arguments.is_a?(String)
+        arguments = if arguments.is_a?(String)
-          arguments = { executable: arguments }
+          { executable: arguments }
         else
           # Avoid mutating the original argument
-          arguments = arguments.dup
+          arguments.dup
         end
 
         # key sanity

Library/Homebrew/test/cask/artifact/abstract_artifact_spec.rb

@@ -0,0 +1,30 @@
+# typed: false
+# frozen_string_literal: true
+
+describe Cask::Artifact::AbstractArtifact, :cask do
+  describe ".read_script_arguments" do
+    it "accepts a string, and uses it as the executable" do
+      arguments = "something"
+      stanza = :installer
+
+      expect(described_class.read_script_arguments(arguments, stanza)).to eq(["something", {}])
+    end
+
+    it "accepts a hash with an executable" do
+      arguments = { executable: "something" }
+      stanza = :installer
+
+      expect(described_class.read_script_arguments(arguments, stanza)).to eq(["something", {}])
+    end
+
+    it "does not mutate the arguments in place" do
+      arguments = { executable: "something", foo: "bar" }
+      clone = arguments.dup
+      stanza = :installer
+
+      described_class.read_script_arguments(arguments, stanza)
+
+      expect(arguments).to eq(clone)
+    end
+  end
+end