False positive failure when comparing metadata with matchingRules

[robpre]

Hi, while comparing metadata using the consumer Matchers.string we found that we were getting peculiar error messages.
Our example produces this error:

 1.1) metadata: Expected 'mock value' (String) to be the same type as 'someString' (String)

The consumer pact generator looks like:

...
  @Pact(consumer = "foo-service")
  public V4Pact exampleForGithub(MessagePactBuilder builder) {
    return builder
        .given("Some State")
        .expectsToReceive("A message with metadata")
        .withMetadata(Map.of("someKey", Matchers.string("someString")))
        .withContent(new PactDslJsonBody()
            .stringType("someField", "someValue")
        )
        .toPact();
  }
...

Produces a pact json like so:

...
      "matchingRules": {
        "body": {...},
        "metadata": {
          "someKey": {
            "combine": "AND",
            "matchers": [{ "match": "type" }]
          }
        }
      },
      "metadata": { "someKey": "someString" },
      "providerStates": [{ "name": "Some State" }],
      "type": "Asynchronous/Messages"
...

I made a branch which has this example in a failing test: master...robpre:pact-jvm:example-error-matching

I ran it down to the mismatch being that MatcherExecutor's function, matchType, compares actual as "mock value" and expected is a JsonValue.StringValue("someString").

Please can you let me know if I've done something wrong here, thanks!