Add initial specs for IO::Buffer #2132

Job	Run time
RuboCop	55s
specs (ubuntu, 3.4.7)	1m 42s
specs (ubuntu, 3.3.9)	1m 31s
specs (macos, 3.2.9)	1m 17s
specs (macos, 3.4.7)	1m 50s
specs (windows, 3.3.9)	3m 5s
specs (windows, 3.4.7)	3m 41s
specs (macos, 3.3.9)	1m 33s
specs (ubuntu, 3.4.7, --enable-frozen-string-literal)	1m 30s
specs (ubuntu, 3.4.7, --parser=parse.y)	1m 40s
specs (windows, 3.2.8)	3m 30s
specs (ubuntu, 3.2.9)	1m 27s
	23m 41s

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