Added Bundle.prototype.isFrozen getter and used it to close pass if it is not yet frozen

src/Bundle.ts

@@ -61,7 +61,7 @@ export default class Bundle {
 	 */
 
 	protected freeze() {
-		if (this[bundleStateSymbol] === BundleState.CLOSED) {
+		if (this.isFrozen) {
 			return;
 		}
 
@@ -69,6 +69,15 @@ export default class Bundle {
 		this[archiveSymbol].end();
 	}
 
+	/**
+	 * Tells if this bundle still allows files to be added
+	 * @returns
+	 */
+
+	public get isFrozen() {
+		return this[bundleStateSymbol] === BundleState.CLOSED;
+	}
+
 	/**
 	 * Allows files to be added to the bundle.
 	 * If the bundle is closed, it will throw an error.
@@ -78,7 +87,7 @@ export default class Bundle {
 	 */
 
 	public addBuffer(fileName: string, buffer: Buffer) {
-		if (this[bundleStateSymbol] === BundleState.CLOSED) {
+		if (this.isFrozen) {
 			throw new Error("Cannot add file. Bundle is closed.");
 		}