The ornamental design for a handheld shower, as shown and described.
FIG. 1 is a perspective view of a handheld shower showing my new design;
FIG. 2 is a bottom end view thereof;
FIG. 3 is a top end view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a right side view thereof, the opposite side being a mirror image of the side shown; and,
FIG. 6 is a front view thereof.
The broken line showing of the face and bottom end of the handheld shower is included for the purpose of illustrating and forms no part of the claimed design.