fix!: isObject now correctly fails for null and undefined

`isObject` was too loose in its definition of what an object is. With
this change, classes, maps, sets, arrays, `null` and `undefined` are no
longer considered objects.

src/expectation/it.ts

@@ -3,6 +3,7 @@ import {
   isEqual,
   isMatchWith,
   isObjectLike,
+  isPlainObject,
   isUndefined,
   omitBy,
 } from 'lodash';
@@ -136,8 +137,12 @@ const isObject = <T extends object, K extends DeepPartial<T>>(
   partial?: K
 ): TypeMatcher<T> =>
   matches(
-    (actual) =>
-      isMatchWith(actual, partial || {}, (argValue, partialValue) => {
+    (actual) => {
+      if (!isPlainObject(actual)) {
+        return false;
+      }
+
+      return isMatchWith(actual, partial || {}, (argValue, partialValue) => {
         if (isMatcher(partialValue)) {
           return partialValue.matches(argValue);
         }

src/expectation/matcher.spec.ts

@@ -599,7 +599,16 @@ describe('It', () => {
   });
 
   describe('isObject', () => {
+    it('should not match non objects', () => {
+      expect(It.isObject().matches('not an object')).toBeFalsy();
+      expect(It.isObject().matches([])).toBeFalsy();
+      expect(It.isObject().matches(null)).toBeFalsy();
+      expect(It.isObject().matches(undefined)).toBeFalsy();
+      expect(It.isObject().matches(new (class {})())).toBeFalsy();
+    });
+
     it('should match any object with empty object', () => {
+      expect(It.isObject().matches({})).toBeTruthy();
       expect(
         It.isObject().matches({
           foo: 'bar',