I think it's just a question of metre. If Shelley had been in need of a syllable to make up a foot, I imagine he would have written 'wouldest'. And I doubt whether he or his type-setter were that worried about apostrophes.
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PS Evidence for this 'metrical' explanation is provided in the first stanza: he uses that full out '-est' form of the verb (albeit a different one) because the metre requires it:Thou wovest dreams of joy and fear
Last edited by BobK; 03-Jan-2011 at 15:24. Reason: PS added