Memory leak and a crash when swizzling NSURLRequest initialiser

When swizzling NSURLRequest initialiser and returning a mutable copy, the original instance does not get deallocated and eventually gets leaked and a crash follows after that.

Here's the swizzling setup:

static func swizzleInit() {
    let initSel = NSSelectorFromString("initWithURL:cachePolicy:timeoutInterval:")

    guard let initMethod = class_getInstanceMethod(NSClassFromString("NSURLRequest"), initSel) else {
        return
    }

    let origInitImp = method_getImplementation(initMethod)

    let block: @convention(block) (AnyObject, Any, NSURLRequest.CachePolicy, TimeInterval) -> NSURLRequest = { _self, url, policy, interval in
        typealias OrigInit = @convention(c) (AnyObject, Selector, Any, NSURLRequest.CachePolicy, TimeInterval) -> NSURLRequest
        let origFunc = unsafeBitCast(origInitImp, to: OrigInit.self)
        let request = origFunc(_self, initSel, url, policy, interval)

        return request.tagged()
    }

    let newImplementation = imp_implementationWithBlock(block as Any)
    method_setImplementation(initMethod, newImplementation)
}

// create a mutable copy if needed and add a header
private func tagged() -> NSURLRequest {
    guard let mutableRequest = self as? NSMutableURLRequest ?? self.mutableCopy() as? NSMutableURLRequest else {
        return self
    }
    mutableRequest.setValue("test", forHTTPHeaderField: "test")
    return mutableRequest
}

Then, we have a few test cases:

// memory leak and crash
func testSwizzleNSURLRequestInit() {
    let request = NSURLRequest(url: URL(string: "https://example.com")!)
    XCTAssertEqual(request.value(forHTTPHeaderField: "test"), "test")
}

// no crash, as the request is mutable, so no copy is created
func testSwizzleNSURLRequestInit2() {
    let request = URLRequest(url: URL(string: "https://example.com")!)
    XCTAssertEqual(request.value(forHTTPHeaderField: "test"), "test")
}

// no crash, as the request is mutable, so no copy is created
func testSwizzleNSURLRequestInit3() {
    let request = NSMutableURLRequest(url: URL(string: "https://example.com")!)
    XCTAssertEqual(request.value(forHTTPHeaderField: "test"), "test")
}

// no crash, as the new instance does not get deallocated 
// when the test method completes (?)
var request: NSURLRequest?
func testSwizzleNSURLRequestInit4() {
    request = NSURLRequest(url: URL(string: "https://example.com")!)
    XCTAssertEqual(request?.value(forHTTPHeaderField: "test"), "test")
}

It appears a memory leak occurs only when any other instance except for the original one is being returned from the initialiser.

Is there a workaround to prevent the leak, while allowing for modifications of all requests?