Use String#encode instead of Iconv

Modify to use String#encode instead of Iconv when ASCII Encodings.
There are two reasons:

1. @iconv was defined only encoding_to is NON_ASCII_ENCODINGS
2. Iconv is replaced from ruby-2.0

But when NON_ASCII_ENCODINGS, String#encoding cannot convert then
use Iconv as in the past.
Merge remote-tracking branch 'upstream/pr/2075'
Use String#encode instead of iconv