fix(timestamp): getTimestamp support times beyond 2038

* Fix getTimestamp for times beyond 2038

Avoid treating timestamps with the most significant bit set as negative. The mongodb documentation describe the timetsamp portion of objectids as 'seconds since the unix epoch', so this seems to be the correct behaviour, although I cannot find a formal specification for it.however this is technically a breaking change if people were relying on timestamp values before 1970.

* use readUInt32BE instead of bit twiddling

* remove comment that no longer applies

* add test case

lib/objectid.js

@@ -249,7 +249,7 @@ class ObjectId {
    */
   getTimestamp() {
     const timestamp = new Date();
-    const time = this.id[3] | (this.id[2] << 8) | (this.id[1] << 16) | (this.id[0] << 24);
+    const time = this.id.readUInt32BE(0);
     timestamp.setTime(Math.floor(time) * 1000);
     return timestamp;
   }

test/node/object_id_tests.js

@@ -99,4 +99,9 @@ describe('ObjectId', function() {
     expect(() => new ObjectId('abcdefghijkl').toHexString()).to.not.throw();
     expect(() => new ObjectId('abcdefŽhijkl').toHexString()).to.throw(TypeError);
   });
+
+  it('should correctly interpret timestamps beyond 2038', function() {
+    var farFuture = new Date('2040-01-01T00:00:00.000Z').getTime();
+    expect(new BSON.ObjectId(BSON.ObjectId.generate(farFuture/1000)).getTimestamp().getTime()).to.equal(farFuture);
+  });
 });