shed_update seems to ignore shed_username in .planemo.yml

I am able to run `planemo shed_create` fine without the `--owner` parameter when `shed_username` is specified in `.planemo.yml`, but when I run `planemo shed_update` I must supply the `--owner` parameter or else I get:
```
Repository [my_nifty_tool] does not exist in the targeted Tool Shed.
Failed to update repository 'my_nifty_tool' as it does not exist on the custom tool shed at localshed.
```

- where I tried this: on my local toolshed (I observed this issue with the testtoolshed but didn't try specifying `--owner` to see whether it worked)
- planemo, version 0.74.7