add model to find the bug of #1976

[zealseeker]

The bug strongloop/loopback#1976 still exists when using relation model to modify a data.
I added an "auser" model which hasMany test. I created a user whose username and password are '123'.
You'd better change the access of User model to make sure the access to auser model. (I changed the node_modules/loopback/common/models/user.json : $everyone permission from DENY to ALLOW )
Do these :

1. add a data from the API of POST /ausers/{id}/test, whose auserId is 1 and data is {"name1":"another"}
2. modify the data from the API of PUT /ausers/{id}/test/{fk} ,whose auserId is 1 , fk is 2 (1 is the data u added in origin) and data is {"name2":"whatever"}
3. you will find that the the name1 will be myDefault automatically with the corresponding log file.