REST search with no language doesn't return results for all languages

[osma]

At which URL did you encounter the problem?

http://api.finto.fi/rest/v1/yso/search?query=bilar

What steps will reproduce the problem?

1. Perform REST search on YSO with Swedish label and no lang parameter

What is the expected output? What do you see instead?

Expected to get results, instead (for above URL) getting this:

{
    "@context": {
    "skos": "http://www.w3.org/2004/02/skos/core#",
    "onki": "http://schema.onki.fi/onki#",
    "uri": "@id",
    "type": "@type",
    "results": 
        {
            "@id": "onki:results",
            "@container": "@list"
        },
        "prefLabel": "skos:prefLabel",
        "altLabel": "skos:altLabel",
        "hiddenLabel": "skos:hiddenLabel",
        "broader": "skos:broader",
        "@language": null
    },
    "uri": "",
    "results": [ ]
}

There are two problems here, probably related:

1. The concept http://www.yso.fi/onto/yso/p1223 is not in the search results, when it should be matched via the Swedish term "bilar".
2. The "@language": null value in the JSON-LD directive is suspicious and probably not what we want.

I tested this also with Skosmos 1.1, it does return the concept and has no @language directive.

[osma]

The problem is related to this line in rest.php:

    $lang = $request->getQueryParam('lang') ? $request->getQueryParam('lang') : $request->getVocab()->getDefaultLanguage();

This doesn't allow not having a lang value as it is forced to the default language if not set.

[osma]

The problem is that changing this to allow empty values will affect all REST methods. For some of them (such as search), not setting lang makes sense. For other it doesn't. These have to be decided on a method-by-method basis based on what the earlier code (Skosmos 1.1) did and also what the REST API documentation says.