The first known documented example seems to be the novel Les Bohémiens, published in 1790 by Anne-Gédéon de Pelleport, a strange Sade-like figure whose fascinating story (and the literary world that inspired his novel) is narrated by Robert Darnton in his article Finding a Lost Prince of Bohemia (I am pretty sure that there is a pdf version on an academic website, somewhere on the Internet). That being said, it seems that this work was completely forgotten; it cannot be enrolled as a “genealogical” ancestor of Puccini's Bohème. However, it does seem that the metaphorical and non-ethnical use of Bohemia(n) was already floating around a century before Puccini.