CSV parser ignores the stream position of a StringIO #99

The CSV parser seems to call @input.string if @input is a StringIO, but I think it should check if @input.pos is zero before doing that.

require 'csv'
require 'stringio'

strio = StringIO.new(<<'EOF')
aaa,b,c
EOF

p strio.read(2) #=> "aa"
p strio.pos #=> 2
p CSV.parse_line(strio) #=> ["aaa", "b", "c"] (["a", "b", "c"] is expected)

This problem was found by @katsyoshi and presented on ruby-jp.slack.com. I tracked it down and identified the causing commit as eeab2ed, which was between v3.0.1 and v3.0.2.