Update assertion of GetFeatureInfo.Valid.InfoFormat

This Pull Request contains a fix for #10 (not tested by me.). The assertion will ignore the response entity and examine the response headers:
 * assert status code = 200 (OK)
 * assert Content-Type matches requested format
 * assert Content-Length > 0

Same fix like Pull Request https://github.com/opengeospatial/ets-wmts10/pull/7.