fails:StringIO#readpartial raises IOError on closed stream
fails:StringIO#readpartial reads at most the specified number of bytes
fails:StringIO#readpartial reads after ungetc with data in the buffer
fails:StringIO#readpartial reads after ungetc without data in the buffer
fails:StringIO#readpartial discards the existing buffer content upon successful read
fails:StringIO#readpartial raises EOFError on EOF
fails:StringIO#readpartial discards the existing buffer content upon error
fails:StringIO#readpartial raises IOError if the stream is closed
fails:StringIO#readpartial raises ArgumentError if the negative argument is provided
fails:StringIO#readpartial immediately returns an empty string if the length argument is 0
