added tests for ReVIEW::Book#prefaces
added tests for ReVIEW::Book#prefaces

masawada/review,orangain/review,orangain/review,suzuki/review,kmuto/review,kdmsnr/review,kuad9/review_study,masawada/review,kuad9/review_study,murashitas/review,murashitas/review,krororo/review,kmuto/review,krororo/review,takahashim/review,suzuki/review,krororo/review,takahashim/review,kmuto/review