make test for Source.buffer more strict

Author: sokra

lib/Compiler.js

@@ -366,7 +366,7 @@ class Compiler extends Tapable {
 							// get the binary (Buffer) content from the Source
 							/** @type {Buffer} */
 							let content;
-							if (source.buffer) {
+							if (typeof source.buffer === "function") {
 								content = source.buffer();
 							} else {
 								const bufferOrString = source.source();