Mirroring effect using OSGB projection

Mirroring effect

When accessing a WMS layer in OSGB36 (British National Grid) projection, the area to the west of Scilly Islands (a group of islands off the westernmost tip of the Great Britain) appears mirrored, featuring a vertically flipped copy of Scilly Islands and South West England in the ocean. The attached screenshot serves as an illustration (true island on the right, mirrored copy on the left).

The MapCache configuration is as follows:
```
   <cache name="bgs_onshore2" type="bdb">
     <base>/bgs_onshore2</base>
   </cache>
   <source name="bgs_onshore2" type="wms">
      <getmap>
         <params>
            <FORMAT>image/png</FORMAT>
            <LAYERS>1</LAYERS>
            <TRANSPARENT>TRUE</TRANSPARENT>
         </params>
      </getmap>
      <http>         <url>http://maps.bgs.ac.uk/ArcGIS/services/BGS_Detailed_Geology/MapServer/WMSServer</url>
         <timeout>3600</timeout>
         <connection_timeout>3600</connection_timeout>
      </http>
   </source>
   <tileset name="bgs_onshore2">
      <metadata>
        <title>BGS Onshore Original</title>
      </metadata>
      <source>bgs_onshore2</source>
      <cache>bgs_onshore2</cache>
      <grid>BNG</grid>
      <format>PNG</format>
      <metatile>5 5</metatile>
      <metabuffer>10</metabuffer>
      <expires>86400</expires>
   </tileset>
```

The seeded tiles are accessed using the following URL:

http://localhost:3005/tiles?LAYERS=bgs_onshore2&TRANSPARENT=true&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&FORMAT=image%2Fpng&SRS=EPSG%3A27700&BBOX=50030.292395313,-6259.7338946798,114893.56276466,30413.824507924&WIDTH=2167&HEIGHT=1225

This has been tested using node mapcache but that as the node bindings simply act as a http shim to the mapcache core it is expected to be replicable in the other mapcache interfaces.

![bgs_mirror](https://f.cloud.github.com/assets/5589208/1249477/128242e2-2ae1-11e3-94d7-c3264343f547.png)