Use mocks to provide MappedByteBuffers.

When called, `FileChannel.map` throws an `UnsupportedOperationException`, but the [Javadoc doesn't mention](http://docs.oracle.com/javase/8/docs/api/java/nio/channels/FileChannel.html#map-java.nio.channels.FileChannel.MapMode-long-long-) any such Exception:

Example code to reproduce the error:

FileSystem fs = Jimfs.newFileSystem();
Path file = fs.getPath("MyFile.data");
FileChannel channel = FileChannel.open(file, READ, WRITE, CREATE, DSYNC);
MappedByteBuffer buffer = channel.map(READ_WRITE, 0L, 1024L);

Okay, I read the code of the method, which is relatively simple:

@Override
public MappedByteBuffer map(MapMode mode, long position, long size) throws IOException {
// would like this to pretend to work, but can't create an implementation of MappedByteBuffer
// well, a direct buffer could be cast to MappedByteBuffer, but it couldn't work in general
throw new UnsupportedOperationException();
}

However, I have to say that I successfully mocked `MappedByteBuffer` using Mockito (latest version). Wouldn't it be possible to create such a mock (using either Mockito or your own bytecode manipulator) so that we can properly test our usage of FileChannel.map?