The ornamental design for a handheld shower, as shown and described.
FIG. 1 is a perspective view of a handheld shower, showing a new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left elevational view thereof;
FIG. 5 is a right elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the handheld shower and form no part of the claimed design.