FIG. 1 is a front, top, and left side perspective view of a soap pump embodying our new design; FIG. 2 is a right side elevational view thereof, the left side elevational view being a mirror image thereof; FIG. 3 is a top plan view thereof; FIG. 4 is a bottom plan view thereof; FIG. 5 is a front elevational view thereof; and, FIG. 6 is a rear elevational view thereof. All features illustrated in phantom line are expressly disclaimed and form no part of the claimed design.