Bug 374442

I can export images from Digikam 5.3.0 to flickr without issue if they have no caption (used for the Flickr description field), or if they have a caption which consists of a single line of text.

However if I have a line break in the caption (typed by pressing enter in the caption field) then the upload always fails with the "Invalid signature" error code from Flickr.

Also reproduced with a build of 5.4.0 from git.

I did some debugging and testing:
* Digikam presents the caption to the plugin as a plain text QString with the line break represented by a line feed character (0xOA).
* Flickr's API documentation for the description just says "A description of the photo. May contain some limited HTML." with no detail on what the limits are. https://www.flickr.com/services/api/upload.api.html

I tried a few different things to see if I could hit on a solution:
* If the newline character is stripped out, then the upload succeeds (so nothing else is wrong).
* If the newline is replaced by a CRLF pair, then the same error occurs
* If the newline is replaced by a CR (only) then the same error occurs
* If the newline is replaced by the HTML <br> tag then the upload succeeds (signature must be OK), but the <br> tag is removed, resulting in a one-line caption with the words either side of the break run together.

My suspicion (but I can't prove it) is that the description text is getting altered before the server-side signature check, which means it doesn't pass - but without knowing what to, we can't get the right signature!