(#5200) - don't clone special objcts like Workers

diff --git a/packages/pouchdb-utils/src/clone.js b/packages/pouchdb-utils/src/clone.js
index 36da3f2..e2d6760 100644
--- a/packages/pouchdb-utils/src/clone.js
+++ b/packages/pouchdb-utils/src/clone.js

@@ -1,5 +1,6 @@
 import isBinaryObject from './isBinaryObject';
 import cloneBinaryObject from './cloneBinaryObject';
+import isPlainObject from './isPlainObject';
 
 function clone(object) {
   var newObject;
@@ -28,6 +29,10 @@
     return cloneBinaryObject(object);
   }
 
+  if (!isPlainObject(object)) {
+    return object; // don't clone objects like Workers
+  }
+
   newObject = {};
   for (i in object) {
     if (Object.prototype.hasOwnProperty.call(object, i)) {

diff --git a/packages/pouchdb-utils/src/isPlainObject.js b/packages/pouchdb-utils/src/isPlainObject.js
new file mode 100644
index 0000000..57a8b33
--- /dev/null
+++ b/packages/pouchdb-utils/src/isPlainObject.js

@@ -0,0 +1,19 @@
+// most of this is borrowed from lodash.isPlainObject:
+// https://github.com/fis-components/lodash.isplainobject/
+// blob/29c358140a74f252aeb08c9eb28bef86f2217d4a/index.js
+
+var funcToString = Function.prototype.toString;
+var objectCtorString = funcToString.call(Object);
+
+function isPlainObject(value) {
+  var proto = Object.getPrototypeOf(value);
+  /* istanbul ignore if */
+  if (proto === null) { // not sure when this happens, but I guess it can
+    return true;
+  }
+  var Ctor = proto.constructor;
+  return (typeof Ctor == 'function' &&
+    Ctor instanceof Ctor && funcToString.call(Ctor) == objectCtorString);
+}
+
+export default isPlainObject;
\ No newline at end of file

diff --git a/tests/unit/test.clone.js b/tests/unit/test.clone.js
new file mode 100644
index 0000000..f81d654
--- /dev/null
+++ b/tests/unit/test.clone.js

@@ -0,0 +1,33 @@
+
+'use strict';
+
+var should = require('chai').should();
+var PouchDB = require('../../packages/pouchdb-for-coverage');
+var clone = PouchDB.utils.clone;
+
+describe('test.clone.js', function () {
+
+  it('Clones regular objects', function () {
+    var obj1 = {foo: 'bar'};
+    var obj2 = clone(obj1);
+    obj1.baz = 'quuz';
+    should.not.exist(obj2.baz);
+  });
+
+  it('Doesn\'t clone fancy objects', function () {
+
+    function Kitty() {
+    }
+
+    Kitty.prototype.meow = function () {
+      return 'meow';
+    };
+
+    var obj1 = {kitty: new Kitty()};
+    var obj2 = clone(obj1);
+    obj1.kitty.meow().should.equal('meow');
+    obj2.kitty.meow().should.equal('meow');
+    obj1.kitty.foo = 'bar';
+    obj2.kitty.foo.should.equal('bar');
+  });
+});