dropboxQt::files::WriteMode::toJson mis-encodes rev in WriteMode_UPDATE case

[barbicels]

[This appears to be a general problem with stone unions that point to a scalar (string, etc.) rather than a struct or other union; for example, dropboxQt::files::LookupError::toJson has the same issue with the malformed_path scalar. The reason why WriteMode poses a problem, and LookupError does not, is that no-one uses dropboxQt to encode a LookupError, because they only occur in results; WriteMode_UPDATE is unique (I think) in being a non-void union that is used in request parameters, via CommitInfo, in requests such as files/upload.]

While writing code to upload a file to Dropbox using dropboxQt::Files::FilesRoutes::upload, using a WriteMode instance tagged WriteMode_UPDATE in CommitInfo to specify the rev of the existing server copy to be updated, I encountered a couple of issues:

1. The WriteMode class has no setter for the m_update (rev) member that is required in the WriteMode_UPDATE union. (Creating a derived class WriteModeUpdate seemed like too much work, so, on to Update ApiUtil.h #2.)
2. I added a setter for its m_update member. Then, when CommitInfo::toJson calls the WriteMode::toJson method, the latter mis-encodes the m_update (rev) value by putting it inside the CommitInfo node as a scalar rather than a tagged node.

This code works for the WriteMode_ADD and WriteMode_OVERWRITE modes only because they are void unions and so it's acceptable to encode mode as a string value inside the CommitInfo node. However, as shown in the Dropbox API v2 Explorer example for files/upload (https://www.dropbox.com/developers/documentation/http/documentation#files-upload), in the WriteMode_UPDATE case, mode has to be a tagged JSON node containing an update (rev) string value. The current dropboxQt code does not produce this; see this screenshot from Qt Creator's debugger, showing the mis-encoded CommitInfo, with an update value beside mode, not inside.

If this were a general stone issue, I would expect it to have been fixed, so could this problem be peculiar to the stone interpreter you used to create dropboxQt?

How should I work around this so that I can program a file upload in WriteMode_UPDATE mode that the server will accept?