BUG: set default styles parameter in GetMap requests

Use "default" for the STYLES parameter of GetMap requests when the parameter is missing or empty. If not set, default-<type>/default (e.g. default-scalar/default) is used by edal-java, which does not take into account defaults set in wmsConfig.xml.

Fixes #552

tds/src/main/java/thredds/server/wms/ThreddsWmsServlet.java

@@ -33,6 +33,7 @@
 import com.google.common.cache.RemovalListener;
 import com.google.common.util.concurrent.UncheckedExecutionException;
 import java.io.IOException;
+import java.util.Collections;
 import java.util.Formatter;
 import java.util.concurrent.ExecutionException;
 import org.slf4j.Logger;
@@ -70,6 +71,8 @@
 public class ThreddsWmsServlet extends WmsServlet {
   private static final Logger logger = LoggerFactory.getLogger(ThreddsWmsServlet.class);
 
+  private static final Map<String, String> defaultStyles = Collections.singletonMap("styles", "default");
+
   private static class CachedWmsCatalogue {
     public final ThreddsWmsCatalogue wmsCatalogue;
     public final long lastModified;
@@ -112,6 +115,11 @@ protected void dispatchWmsRequest(String request, RequestParams params, HttpServ
     TdsRequestedDataset tdsDataset = new TdsRequestedDataset(httpServletRequest, removePrefix);
     ThreddsWmsCatalogue catalogue = acquireCatalogue(httpServletRequest, httpServletResponse, tdsDataset.getPath());
 
+    // set default style if needed
+    if (request.equals("GetMap") && params.getString("styles", "").isEmpty()) {
+      params = params.mergeParameters(defaultStyles);
+    }
+
     /*
      * Now that we've got a WmsCatalogue, we can pass this request to the
      * super implementation which will handle things from here.