Encode using utf-8 when casting to string

[mrosanes]

Fix bug. Encode models using utf-8, instead of a hard cast
to string, in order to allow curves addition to a plot.

[mrosanes]

Hi @asenchenko; this pull request, together with a minor change proposed to taurus taurus-org/taurus#878, should allow to add curves to a taurus_pyqtgraph plot with python2 and python3.

If you agree with it, could you integrate it?

[asenchenko]

Hi @mrosanes! Thank you. PR taurus-org/taurus/pull/878 fixes problem with adding curve to plot.
As i understand, your PR should solve problem with empty table in Curve Selection dialog.
I think that QByteArray should be casted to bytes and than to unicode.

[cpascual]

I think that QByteArray should be casted to bytes and than to unicode.

I agree with @asenchenko. His commit seems a better approach to me. So I suggest to close this PR