Inconsistent API behavior when using `deleted=0`

[boiko]

Issue Description

The API endpoint that list project contents has an inconsistent behavior when explicitly passing deleted=0 if the project has multibuild packages, see this example:

> osc api /source/devel:languages:python:Factory | grep python36
  <entry name="python36"/>

Now if I append deleted=0 to the arguments, the API endpoint lists also multibuild variants:

> osc api /source/devel:languages:python:Factory?deleted=0 | grep python36
  <entry name="python36"/>
  <entry name="python36:base" originpackage="python36"/>
  <entry name="python36:doc" originpackage="python36"/>

Expected Result

Setting deleted=0 should not change the behavior or the API endpoint and return the same results as if the argument was not passed at all.