Use lodash assign
Use lodash assign

Fixes https://github.com/samselikoff/ember-cli-mirage/issues/979

`Object.assign` is not supported by PhantomJS, which is the default test
runner for ember apps

samselikoff/ember-cli-mirage,samselikoff/ember-cli-mirage,jherdman/ember-cli-mirage,unchartedcode/ember-cli-mirage,makepanic/ember-cli-mirage,unchartedcode/ember-cli-mirage,jherdman/ember-cli-mirage,mixonic/ember-cli-mirage,jherdman/ember-cli-mirage,cs3b/ember-cli-mirage,unchartedcode/ember-cli-mirage,samselikoff/ember-cli-mirage,mixonic/ember-cli-mirage,cs3b/ember-cli-mirage,makepanic/ember-cli-mirage,cibernox/ember-cli-mirage,unchartedcode/ember-cli-mirage,jherdman/ember-cli-mirage,samselikoff/ember-cli-mirage,cibernox/ember-cli-mirage