Fix 0013007: Moving an issue to a sub-project - default category used

Assume project B is a sub-project of project A.

When moving an issue with category C (which is assigned to the project
A) to project B, the category gets changed to DEFAULT (configured
default category) and doesn't respect the fact, that project B is a
subproject of A and does inherit the category C.

Steps to reproduce this bug:

1. Create two projects: A and B. B is subproject of A
2. Create category C in project A so that B inherits the category.
3. Create issue I in project A
4. Move I to project B
5. Note that I has the default category instead of category C

Signed-off-by: David Hicks d@hx.id.au