Mattress

FIG. 1 is a front elevation view of a mattress showing the new design; FIG. 2 is a rear elevation view thereof; FIG. 3 is a left side elevation view thereof; FIG. 4 is a right side elevation view thereof; FIG. 5 is a top plan view thereof; FIG. 6 is a bottom plan view thereof; FIG. 7 is a first perspective view thereof; and, FIG. 8 is a second perspective view thereof. The broken lines shown in the drawing figures are included for the purpose of illustrating portions of the mattress that form no part of the claimed design.