fails:StringIO#readline when passed [separator] returns the data read till the next occurence of the passed separator
fails:StringIO#readline when passed [separator] sets $_ to the read content
fails:StringIO#readline when passed [separator] updates self's lineno by one
fails:StringIO#readline when passed [separator] returns the next paragraph when the passed separator is an empty String
fails:StringIO#readline when passed [separator] returns the remaining content starting at the current position when passed nil
fails:StringIO#readline when passed [separator] tries to convert the passed separator to a String using #to_str
fails:StringIO#readline when in write-only mode raises an IOError
