The ornamental design for a shower fitting assembly, as shown and described.
FIG. 1 is a top view of my shower fitting assembly, the dotted line portion of which is not a feature of my design, but which is included to provide a better understanding of my design as it is used and forms no part of the claimed design;
FIG. 2 is a right side view of my shower fitting assembly (the left side view being a mirror image thereof);
FIG. 3 is a bottom view of my shower fitting assembly;
FIG. 4 is a rear view of my shower fitting assembly; and,
FIG. 5 is a front view of my shower fitting assembly.