Adding zoomselector test

test/rendering/zoomselector.mml

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+{
+    "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+    "Stylesheet": [
+        "zoomselector.mss"
+    ],
+    "Layer": [{
+        "name": "world",
+        "id": "world",
+        "class": "",
+        "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+        "Datasource": {
+            "file": "http://tilemill-data.s3.amazonaws.com/test_data/shape_demo.zip",
+            "type": "shape"
+        }
+    }]
+}

test/rendering/zoomselector.mss

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+#world {
+    [zoom>9] { marker-height: 4; }
+    [zoom>6][zoom<10] { marker-height: 3; }
+    [zoom>3][zoom<7] { marker-height: 2; }
+    [zoom<4] { marker-height: 1; }
+}

test/rendering/zoomselector.result

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+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE Map[]>
+<Map srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
+
+
+<Style name="world" filter-mode="first">
+  <Rule>
+    <MaxScaleDenominator>50000000</MaxScaleDenominator>
+    <MinScaleDenominator>6500000</MinScaleDenominator>
+    <MarkersSymbolizer height="2" />
+  </Rule>
+  <Rule>
+    <MaxScaleDenominator>6500000</MaxScaleDenominator>
+    <MinScaleDenominator>750000</MinScaleDenominator>
+    <MarkersSymbolizer height="3" />
+  </Rule>
+  <Rule>
+    <MinScaleDenominator>50000000</MinScaleDenominator>
+    <MarkersSymbolizer height="1" />
+  </Rule>
+  <Rule>
+    <MaxScaleDenominator>750000</MaxScaleDenominator>
+    <MarkersSymbolizer height="4" />
+  </Rule>
+</Style>
+<Layer
+      name="world"
+   srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
+    <StyleName>world</StyleName>
+    <Datasource>
+       <Parameter name="file"><![CDATA[[absolute path]]]></Parameter>
+       <Parameter name="type"><![CDATA[shape]]></Parameter>
+    </Datasource>
+  </Layer>
+
+</Map>