Fix String#append_as_bytes to accept Integer

[bdewater-thatch]

See rdoc example a few lines above the diff, this is valid too.

rbs/core/string.rbs

Lines 1620 to 1622 in 5771976

	# s = ""
	# s.append_as_bytes(0, 257) # => "\u0000\u0001"
	# s.bytesize # => 2