Remove dependency on encoding class

[Motoshi-Nishihira]

fix broken build about #251

Encoding class are implemented as of ruby 1.9.

In ruby 1.8, string don't have the encoding information.
So no error is raised by joining different encoding strings.

irb(main):001:0> ["∞".kconv(Kconv::EUC, Kconv::UTF8), "∞"].join
=> "\241\347\342\210\236"

As of 1.9

irb(main):001:0> ["∞".kconv(Kconv::EUC, Kconv::UTF8), "∞"].join
Encoding::CompatibilityError: incompatible character encodings: EUC-JP and UTF-8

[sferik]

It looks like some of our code quality metrics are now broken. Would you mind refactoring this pull request to fix those?

[coveralls]

Coverage increased (+0.07%) to 96.629% when pulling ee7d5d5 on Motoshi-Nishihira:remove-dependency-on-encoding-class into 7691a2c on intridea:master.

[sferik]

Thanks!