You raise a fair point about bias in covering "fringe" candidates, which the Times has been very slow to get past with Sanders. He's a mainstream candidate now who has to be taken seriously, with a clear and strong voice that is influencing Clinton's policy statements to reflect his, while at the same time she hasn't had nearly as much impact on his. The Times editorial watchdog (Margaret Sullivan) agrees that the unusually significant changes to the original article's content and positive tone caused the revised article to reflect the Times editorial view, which has been less flattering.
I read the article you linked to and all of the articles it links to, as well. I also agree that the Times introduced a point of view (which they're entitled to do), but there is one section in the original Steinhauer article that is consistent with the Times overall editorial perspective that is overlooked in the discussion:
Big legislation largely eludes Mr. Sanders because his ideas are usually far to the left of the majority of the Senate — from his notions about bank regulations, to the increase he seeks to the minimum wage, to his repeated attempts to get the federal government in the business of providing rebates for the purchase and installation of solar heating systems.
But from his days in the House, where he served from 1991 to 2007, and into his Senate career, Mr. Sanders has largely found ways to press his agenda through appending small provisions to the larger bills of others.
To paraphrase, he operates (and succeeds) at the margins because his larger ideas are not consistent with either Party's agenda. You could argue that he has been effective, but you can also argue that while his changes have in some cases had a significant impact on legislation, he has not been a leader on program or policy development in either the House or Senate. The changes made to the article put more emphasis on this aspect of his legislative history, which if heightening the Times editorial point of view, is not inaccurate.