improve test coverage for Dumper
improve test coverage for Dumper

joy32812/snakeyaml,Mause/snakeyaml,ldh0826/snakeyaml,yeison/snakeyaml,UcasRichard/snakeyaml,Akkrite/snakeyaml,RussellSpitzer/snakeyaml,nxbdi/snakeyaml,youxinren/snakeyaml,mc-nekoneko/snakeyaml,AssafMashiah/assafmashiah-yaml,umeding/snakeyaml,yishenggudou/snakeyaml,ilantoren/snakeyaml,marcuskara/snakeyaml,Fighter777/snakeyaml,yskumar007/snakeyaml,thinkofdeath/snakeyaml,votinhthieugia/snakeyaml,SorinS/snakeyaml,jeremypepper/snakeyaml,mapiman/snakeyaml,virustotalop/snakeyaml,cuizhennan/snakeyaml