In a blender in batches or in a food processor puree the honeydew, the mint leaves, the lime juice, the sugar to taste, and a pinch of salt until the mixture is smooth, transfer the soup to a bowl, and chill it, covered, for a least 1 hour or overnight. The soup may be made 2 days in advance and kept covered and chilled. Arrange the cantaloupe halves on 6 small plates, ladle the soup into them, and garnish each serving with a mint sprig.