Language server middleware occasionally fails to publish diagnostics

[MikhailArkhipov]

LanguageClientMiddleware is implemented as a class and relies on this. However, in VSC client middleware is assumed to be function at least for handleDiagnostics i.e.

handleDiagnostics?: (this: void, uri: Uri, diagnostics: VDiagnostic[], next: HandleDiagnosticsSignature) => void;

and

        let middleware = this.clientOptions.middleware.handleDiagnostics;
        if (middleware) {
            middleware(uri, diagnostics, (uri, diagnostics) => this.setDiagnostics(uri, diagnostics));
        }

This causes diagnostics reports to fail b.c this is undefined.