Don't call toJSON() when formatting values

Fixes #1233. Our assertions don't call toJSON(), so we shouldn't use
it either when displaying actual and expected values.

lib/serialize-error.js

@@ -9,6 +9,7 @@ const extractStack = require('./extract-stack');
 
 function serializeValue(value) {
 	return prettyFormat(value, {
+		callToJSON: false,
 		plugins: [reactTestPlugin],
 		highlight: true
 	});

test/serialize-error.js

@@ -17,6 +17,7 @@ sourceMapSupport.install({environment: 'node'});
 
 function serializeValue(value) {
 	return prettyFormat(value, {
+		callToJSON: false,
 		plugins: [reactTestPlugin],
 		highlight: true
 	});
@@ -183,3 +184,29 @@ test('skip actual and expected if output is off', t => {
 	t.notOk(serializedErr.expectedType);
 	t.end();
 });
+
+test('does not call toJSON() when serializing actual and expected', t => {
+	const err = Object.assign(new Error(), {
+		showOutput: true,
+		actual: {
+			foo: 'bar',
+			toJSON() {
+				return {
+					foo: 'BAR'
+				};
+			}
+		},
+		expected: {
+			foo: 'thud',
+			toJSON() {
+				return {
+					foo: 'BAR'
+				};
+			}
+		}
+	});
+
+	const serializedErr = serialize(err);
+	t.notSame(serializedErr.actual, serializedErr.expected);
+	t.end();
+});