Add test for JSON-based cask config loader

Previously, the JSON-based cask config loader was untested.
This commit changes the interface to accept a string, making the loader
easier to test. The commit also adds a test.

Library/Homebrew/cask/config.rb

@@ -34,15 +34,15 @@ module Cask
 
     def self.for_cask(cask)
       if cask.config_path.exist?
-        from_file(cask.config_path)
+        from_json(File.read(cask.config_path))
       else
         global
       end
     end
 
-    def self.from_file(path)
+    def self.from_json(json)
       config = begin
-        JSON.parse(File.read(path))
+        JSON.parse(json)
       rescue JSON::ParserError => e
         raise e, "Cannot parse #{path}: #{e}", e.backtrace
       end

Library/Homebrew/test/cask/config_spec.rb

@@ -3,6 +3,21 @@
 describe Cask::Config, :cask do
   subject(:config) { described_class.new }
 
+  describe "::from_json" do
+    it "deserializes a configuration in JSON format" do
+      config = described_class.from_json <<~EOS
+        {
+          "default": {
+            "appdir": "/path/to/apps"
+          },
+          "env": {},
+          "explicit": {}
+        }
+      EOS
+      expect(config.appdir).to eq(Pathname("/path/to/apps"))
+    end
+  end
+
   describe "#default" do
     it "returns the default directories" do
       expect(config.default[:appdir]).to eq(Pathname(TEST_TMPDIR).join("cask-appdir"))