[PHP-627] Allow single pipeline operator arg in MongoCollection::aggregate()

When running standalone tests (ignoring sharding and replica sets):

```
=====================================================================
FAILED TEST SUMMARY
---------------------------------------------------------------------
Test for PHP-207: setSlaveOkay not supported for GridFS queries [tests/generic/bug00207.phpt]
MongoGridFSFile::write() [tests/generic/mongogridfsfile-write-001.phpt]
Test for PHP-270: ext/mongo classes should return meaningful results from Reflection API [tests/others/bug00270-arginfo.phpt]
Test for MongoLog [tests/standalone/log-1.phpt]
Test for MongoLog (connection only) [tests/standalone/log-2.phpt]
Test for MongoLog (level variations) [tests/standalone/log-4.phpt]
=====================================================================
```