Fix RDAP quirks uncovered during documentation

There's no reason not to allow a one-character search string when there are no wildcards. And the ROID validity pattern did not allow underscores, which was causing problems with our ROIDs.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136256605

java/google/registry/rdap/RdapEntityAction.java

@@ -49,7 +49,7 @@ public class RdapEntityAction extends RdapActionBase {
 
   public static final String PATH = "/rdap/entity/";
 
-  private static final Pattern ROID_PATTERN = Pattern.compile("[-.a-zA-Z0-9]+");
+  private static final Pattern ROID_PATTERN = Pattern.compile("[-_.a-zA-Z0-9]+");
 
   @Inject Clock clock;
   @Inject RdapEntityAction() {}