Fix specifying of additional attributes on pictures

[jntullo]

Encoding errors strike again!

Requesting any additional attributes for a picture results in:

error:
{
    "error": {
        "kind": "internal_server_error",
        "message": "\"\\x89\" from ASCII-8BIT to UTF-8",
        "klass": "Encoding::UndefinedConversionError"
    }
}

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1547852

[abellotti]

somehow with this PR, asking for an attribute I get them all, i.e. GET /api/pictures/2?attributes=extension, I get them all instead of just id/href/extension as we do with master.

[abellotti]

now I see all attributes with GET /api/pictures without attributes specified instead of just the href.

[miq-bot]

Checked commit jntullo@2ae5d78 with ruby 2.3.3, rubocop 0.52.0, haml-lint 0.20.0, and yamllint 1.10.0
3 files checked, 0 offenses detected
Everything looks fine. 🍰

[abellotti]

LGTM!! Thanks @jntullo for updating/fixing this. 👍

[simaishi]

Gaprindashvili backport details:

$ git log -1
commit 7fb24195cffa6da14b0b980bac87747a4e5c0d0c
Author: Alberto Bellotti <abellotti@users.noreply.github.com>
Date:   Tue Feb 27 10:26:39 2018 -0500

    Merge pull request #327 from jntullo/bz_1547852
    
    Fix specifying of additional attributes on pictures
    (cherry picked from commit a8d5c3ac6ae862000a4b71b0517dd85275beb326)
    
    Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1552828