Fix JsonReaderTest implementation issues
Fix JsonReaderTest implementation issues

* Replace use of Java 8+ Function interface with our own. The driver
still runs tests on Java 6, so we can't use Java 8 APIs even in tests
* Specify Charset for InputStreamReader

rozza/mongo-java-driver,rozza/mongo-java-driver,jyemin/mongo-java-driver,jyemin/mongo-java-driver