Description:
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Using any argument list parameter
The separator disappears
For using price ASC for instance
the end result is priceasc

Test script:
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$dismaxQuery = new SolrDisMaxQuery('score');
$dismaxQuery->addSortField('price', SolrQuery::ORDER_ASC);
$dismaxQuery->addSortField('score', SolrQuery::ORDER_DESC);
echo $dismaxQuery.PHP_EOL;

Expected result:
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q=score&sort=price asc,score desc

Actual result:
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q=score&sort=priceasc,score desc