fix for duplicated URL params in wmslegend

This PR fixes the broken behaviour in GeoExt.container.WmsLegend, when baseParams are used.
Before this PR, when using baseParams on instanciation like described in the documentation http://geoext.github.io/geoext2/docs/#!/api/GeoExt.container.WmsLegend-cfg-baseParams and you set the 'EXCEPTIONS' parameter as baseParam to a different value, multiple EXCEPTIONS-parameters will appear in the final URL. 
Example:
```
/ows?SERVICE=WMS&VERSION=1.1.1&REQUEST=GetLegendGraphic&EXCEPTIONS=application%2Fvnd.ogc.se_inimage&FORMAT=image%2Fpng&EXCEPTIONS=application%2Fvnd.ogc.se_xml&LAYER=a&foo=bar%20bar
```
This was happening because of the usage of Ext.urlAppend with parameteres which have not been cleanly 'merged' with Ext.apply.
This fix handles this case and applies parameters, so that they dont get duplicated in the final request url.
Test has also been fixed to reflect the change in order of the params.