Flushing DataOutputStream before calling toByteArray on the underlying ByteArrayOutputStream

[emopers]

EndianSwitchableOutputStream.java internally uses DataOutputStream as its stream and when a DataOutputStream instance wraps an underlying ByteArrayOutputStream instance, it is recommended to flush or close the DataOutputStream before invoking the underlying instances's toByteArray(). Also, it is a good practice to call flush/close explicitly as mentioned for example here. This pull request adds a flush method before calling toByteArray().