Fix to avoid incompatibility if compile with jdk9

[nroduit]

Java 9 introduces overridden methods with covariant return types for the following methods in java.nio.ByteBuffer:

• position​(int newPosition)
• limit​(int newLimit)
• flip​()
• clear​()
• mark​()
• reset​()
• rewind​()

In Java 9 they all now return ByteBuffer, whereas the methods they override return Buffer,
resulting in exceptions like this when executing on Java 8 and lower:
java.lang.NoSuchMethodError: java.nio.ByteBuffer.limit(I)Ljava/nio/ByteBuffer
This is because the generated byte code includes the static return type of the method, which is not found on Java 8 and lower because the overloaded methods with covariant return types don't exist (the issue appears even with source and target 8 or lower in compilation parameters).
The solution is to cast ByteBuffer instances to Buffer before calling the method.