Update assertion of GetTile.Valid.Tile.Format

This Pull Request contains a fix for #14.


Only ```KVP.GetTile.Valid.Tile.Format``` was tested with [deegree 3.4-pre15] (http://cite.lat-lon.de/deegree-webservices-3.4-pre15/services/wmts?service=WMTS&request=GetCapabilities).


The assertion will will check, if the response image format is like request image format (with ```contains```). Now the test is passed while requesting with ```image/png8```.