GUACAMOLE-422: Correct toString() for GuacamoleProtocolVersion (must be manually implemented after migrating away from enum). Add unit test for verifying toString().

guacamole-common/src/main/java/org/apache/guacamole/protocol/GuacamoleProtocolVersion.java

@@ -202,5 +202,10 @@ public class GuacamoleProtocolVersion {
             && this.patch == otherVersion.getPatch();
 
     }
+
+    @Override
+    public String toString() {
+        return "VERSION_" + getMajor() + "_" + getMinor() + "_" + getPatch();
+    }
     
 }