polygon-pattern-file fails when the url has query parameters

[javisantana]

try to create a layergroup using this:

polygon-pattern-file: url(http://libs.cartocdn.com.s3.amazonaws.com/stamen-base/park-halftone-1.png?h=d68def69037325f4c11a0316e4c49df3);

it raises:

{"errors":["file could not be found: '/home/ubuntu/tile_assets/cache/1d281935-park-halftone-1.png?h=d68def69037325f4c11a0316e4c49df3' in style 'layer0' in PolygonPatternSymbolizer"]}

[jchamberlain]

Would love to see this fixed. Just left a description of our use case on your PR at tilemill-project/millstone#117.