Add text/html to advertised GetFeatureInfo formats

[hugcis]

This PR addresses this issue: #188

As far as I understand, this was only a matter of advertising the format in GetCapabilities, since the actual request is forwarded to the backend WMS server.

[jmckenna]

@hugcis thanks, can you also add that format to /tests/expected/wms_capabilities.xml ?

[jratike80]

Excuse me, but if these are the default formats

  <Format>text/plain</Format>
  <Format>application/vnd.ogc.gml</Format>

does that mean that they are always included in the GetCapabilities? Because the WMS standard does not mandate any format for GetFeatureInfo, the formats should actually be configured for each layer independently. But I do not see such configuration possibility in https://mapserver.org/mapcache/config.html.