Revert r45670 "stringio.c: use rb_str_append"

It breaks test-all.
http://u64b.rubyci.org/~chkbuild/ruby-trunk/log/20140422T143301Z.diff.html.gz

rb_str_conv_enc() return original string for example when
from ASCII-8BIT to UTF-8 with non ASCII strings

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45674 b2dd03c8-39d4-4d8f-98ff-823fe69b080e