fix: update REST transport error details

[alicejli]

Currently, if invoking a PHP client library via REST transport (e.g. $newClient = new ServiceClient(['transport' => 'rest']);), the error details displayed are serialized improperly resulting in gibberish (or close to it) error messages.
For instance, the error message currently if invoking a service that a cloud project is not enabled for looks like this:

Google\ApiCore\ApiException: {
    "message": "Cloud Translation API has not been used in project 372459398382 before or it is disabled. Enable it by visiting https:\/\/console.developers.google.com\/apis\/api\/translate.googleapis.com\/overview?project=372459398382 then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry.",
    "code": 7,
    "status": "PERMISSION_DENIED",
    "details": [
        {
            "@type": 0,
            "data": "type.googleapis.com\/google.rpc.Help"
        },
        {
            "@type": 0,
            "data": [
                {
                    "description": "Google developers console API activation",
                    "url": "https:\/\/console.developers.google.com\/apis\/api\/translate.googleapis.com\/overview?project=372459398382"
                }
            ]
        },
        {
            "@type": 1,
            "data": "type.googleapis.com\/google.rpc.ErrorInfo"
        },
        {
            "@type": 1,
            "data": "SERVICE_DISABLED"
        },
        {
            "@type": 1,
            "data": "googleapis.com"
        },
        {
            "@type": 1,
            "data": {
                "service": "translate.googleapis.com",
                "consumer": "projects\/372459398382"
            }
        }
    ]
}

With this fix, the error message will instead look like this:

Google\ApiCore\ApiException: {
    "message": "Cloud Translation API has not been used in project 372459398382 before or it is disabled. Enable it by visiting https:\/\/console.developers.google.com\/apis\/api\/translate.googleapis.com\/overview?project=372459398382 then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry.",
    "code": 7,
    "status": "PERMISSION_DENIED",
    "details": [
        {
            "@type": "type.googleapis.com\/google.rpc.Help",
            "links": [
                {
                    "description": "Google developers console API activation",
                    "url": "https:\/\/console.developers.google.com\/apis\/api\/translate.googleapis.com\/overview?project=372459398382"
                }
            ]
        },
        {
            "@type": "type.googleapis.com\/google.rpc.ErrorInfo",
            "reason": "SERVICE_DISABLED",
            "domain": "googleapis.com",
            "metadata": {
                "consumer": "projects\/372459398382",
                "service": "translate.googleapis.com"
            }
        }
    ]
}

This technically could be considered a breaking change, so we may need to bundle this change into a new version. However, the current behavior is clearly buggy so not sure if this needs to be considered a breaking change.

[bshaffer]

This looks great! Just two minor tweaks, and also please add some tests for this.