function Polar2D(r, tha) {
      this.r = Math.abs(r) || 0;
      this.tha = tha || 0;
    }
converts a rgb value to a rgb string

@memberof Proton#Proton.Util
@method rgbToHex

@param {Object | Proton.hexToRgb} rgb a rgb object like in {@link Proton#Proton.}

@return {String} rgb()