GCS_MAVLink: fixed bug setting parameters to default values

in copter if you try to set RATE_RLL_D to 0 when you haven't
prevviously changed it then it would set it, but would revert on the
next reboot. This is because of the special case handling of a set to
the "default" value. That default value is unaware of the PID
constructors

this fixes that behaviour by forcing a save if the parameter changes
value

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -556,13 +556,27 @@ void GCS_MAVLINK::handle_param_set(mavlink_message_t *msg, DataFlash_Class *Data
     strncpy(key, (char *)packet.param_id, AP_MAX_NAME_SIZE);
     key[AP_MAX_NAME_SIZE] = 0;
 
-    vp = AP_Param::set_param_by_name(key, packet.param_value, &var_type);
+    // find existing param so we can get the old value
+    vp = AP_Param::find(key, &var_type);
     if (vp == NULL) {
         return;
     }
+    float old_value = vp->cast_to_float(var_type);
+
+    // set the value
+    vp->set_float(packet.param_value, var_type);
+
+    /*
+      we force the save if the value is not equal to the old
+      value. This copes with the use of override values in
+      constructors, such as PID elements. Otherwise a set to the
+      default value which differs from the constructor value doesn't
+      save the change
+     */
+    bool force_save = !is_equal(packet.param_value, old_value);
 
     // save the change
-    vp->save();
+    vp->save(force_save);
 
     // Report back the new value if we accepted the change
     // we send the value we actually set, which could be