GWC REST API fix to fill in the TileLayerInfo blanks when creating a TileLayer

[groldan]

To be raised as an issue upstream. Work around the behaivor
mismatch between GWC's REST API and GeoServer's GWC web-ui.

When creating a tile layer with the web-ui, the GeoServerTileLayerInfo
is populated with the defaults from GWCConfig (set in GWCSettings page).

When creating a tile layer through the REST API, it allows a minimal
payload but does not fill in the blanks with the defaults from GWConfig.

e.g., the following payload:

<GeoServerLayer><enabled>true</enabled><name>topp:states</name></GeoServerLayer>

results in an unsusable tile layer:

<?xml version="1.0" encoding="UTF-8"?>
<GeoServerLayer>
  <id>LayerInfoImpl-8fbb6079-2ee3-40b0-955a-2ccf1b8c2941</id>
  <enabled>true</enabled>
  <name>topp:states</name>
  <mimeFormats/>
  <gridSubsets/>
  <metaWidthHeight>
    <int>0</int>
    <int>0</int>
  </metaWidthHeight>
  <expireCache>0</expireCache>
  <expireClients>0</expireClients>
  <parameterFilters/>
  <gutter>0</gutter>
  <cacheWarningSkips/>
</GeoServerLayer>

where from the default tile layer settings it should be more like:

<GeoServerLayer>
  <id>LayerInfoImpl-8fbb6079-2ee3-40b0-955a-2ccf1b8c2941</id>
  <enabled>true</enabled>
  <name>topp:states</name>
  <mimeFormats>
    <string>image/png</string>
    <string>image/jpeg</string>
  </mimeFormats>
  <gridSubsets>
    <gridSubset>
      <gridSetName>EPSG:4326</gridSetName>
    </gridSubset>
    <gridSubset>
      <gridSetName>EPSG:900913</gridSetName>
    </gridSubset>
  </gridSubsets>
  <metaWidthHeight>
    <int>4</int>
    <int>4</int>
  </metaWidthHeight>
  <expireCache>0</expireCache>
  <expireClients>0</expireClients>
  <parameterFilters>
    <styleParameterFilter>
      <key>STYLES</key>
      <defaultValue></defaultValue>
    </styleParameterFilter>
  </parameterFilters>
  <gutter>0</gutter>
  <cacheWarningSkips/>
</GeoServerLayer>