FileOutboundChannelAdapterIntegrationTests does not pass on Windows

The main issue is lying on File.separator which is different on different OSs.
FileOutboundChannelAdapterIntegrationTests#saveToSubDirWithWrongExpression throws on Windows:

java.lang.IllegalArgumentException: Destination path [target\base-directory\sub-directory\foo.txt] does not point to a directory.

which isn't the same what expected:

Destination path [target/base-directory/sub-directory/foo.txt] does not point to a directory.