Revert the removal of UTF-8 force encoding in JSON loading

I think the removal of the force encoding in #719 was a mistake. For example, my shell prompt includes multi-byte UTF-8 characters, and when `Spring::Client::Run#run_command` collects the environment variables using `ENV.to_hash`, the resulting JSON string becomes ASCII-8BIT. This causes issues when the `Spring::Application#serve` tries to load the JSON string and passes it to `JSON.load`. Since `OkJson` expects UTF-8 encoded strings, it raises an error when it encounters the ASCII-8BIT string.

I've added a test case that replicates this issue to ensure that the encoding is handled correctly. The test case creates a JSON string with multi-byte UTF-8 characters as an ASCII-8BIT string and verifies that it can be loaded without raising an error.

Author: paracycle

lib/spring/json.rb

@@ -13,6 +13,7 @@
 module Spring
   module JSON
     def self.load(string)
+      string = string.dup.force_encoding("utf-8") unless string.encoding == Encoding::UTF_8
       OkJson.decode(string)
     end
 

test/unit/json_test.rb

@@ -6,7 +6,12 @@ class JsonTest < ActiveSupport::TestCase
     assert_equal({"unicode_example"=>"©"}, Spring::JSON.load('{"unicode_example": "\u00A9"}'))
   end
 
+  test 'can decode binary strings with valid UTF8 characters' do
+    string = "{\"PS1\":\"\xEF\x90\x98 main \xEE\x9E\x91 v3.4.2\"}".b
+    assert_equal({"PS1"=>" main  v3.4.2"}, Spring::JSON.load(string))
+  end
+
   test 'can encode' do
     assert_equal('{}', Spring::JSON.dump({}))
-  end 
+  end
 end