Prevent prototype pollution on baseUnset function

* test: add tests to prevent security regressions

* sec: prevent prototype pollution on `baseUnset` function

* chore: improve security patch

- Expand both `_.omit` & `_.unset` security tests to loop over `__proto__`, `constructor`, `prototype`
- Only block `__proto__` if not an own property

lodash.js

@@ -4370,8 +4370,47 @@
      */
     function baseUnset(object, path) {
       path = castPath(path, object);
-      object = parent(object, path);
-      return object == null || delete object[toKey(last(path))];
+
+      // Prevent prototype pollution, see: https://github.com/lodash/lodash/security/advisories/GHSA-xxjr-mmjv-4gpg
+      var index = -1,
+          length = path.length;
+
+      if (!length) {
+        return true;
+      }
+
+      var isRootPrimitive = object == null || (typeof object !== 'object' && typeof object !== 'function');
+
+      while (++index < length) {
+        var key = path[index];
+
+        // skip non-string keys (e.g., Symbols, numbers)
+        if (typeof key !== 'string') {
+          continue;
+        }
+
+        // Always block "__proto__" anywhere in the path if it's not expected
+        if (key === '__proto__' && !hasOwnProperty.call(object, '__proto__')) {
+          return false;
+        }
+
+        // Block "constructor.prototype" chains
+        if (key === 'constructor' &&
+            (index + 1) < length &&
+            typeof path[index + 1] === 'string' &&
+            path[index + 1] === 'prototype') {
+
+          // Allow ONLY when the path starts at a primitive root, e.g., _.unset(0, 'constructor.prototype.a')
+          if (isRootPrimitive && index === 0) {
+            continue;
+          }
+
+          return false;
+        }
+      }
+
+      var obj = parent(object, path);
+      return obj == null || delete obj[toKey(last(path))];
     }
 
     /**