Use Chai to perform object diffs based on the config

lib/chai-subset.js

@@ -1,14 +1,20 @@
 var CircularJSON = require('circular-json');
 
-module.exports = function(chai) {
-	chai.Assertion.addChainableMethod('containSubset', function (expected) {
-		var actual = this.__flags.object;
-		var msg = "\n" + CircularJSON.stringify(actual, null, "\t") + "\n";
-		this.assert(
+module.exports = function(chai, utils) {
+	var Assertion = chai.Assertion;
+	var assertionPrototype = Assertion.prototype;
+
+	Assertion.addChainableMethod('containSubset', function (expected) {
+		var actual = utils.flag(this, 'object');
+		var showDiff = chai.config.showDiff;
+
+		assertionPrototype.assert.call(this,
 			compare(expected, actual),
-				'expected' + msg + 'to contain subset \n#{exp}',
-				'expected' + msg + 'not to contain subset \n#{exp}',
-			expected
+			'expected #{act} to contain subset #{exp}',
+			'expected #{act} to not contain subset #{exp}',
+			expected,
+			actual,
+			showDiff
 		);
 	});
 };
@@ -44,4 +50,4 @@ function compare(expected, actual) {
 		}
 		return ao === eo;
 	});
-}
+}