Make Encoding::Converter#inspect consistent with MRI

Hello,

This pull request aims at making `Encoding::Converter#inspect` consistent with the output on MRI. On the latter, the method returns both the source and destination encodings. The method was previously not defined on the Rubinius side.

Have a nice day.
The description could probably be a bit more specific in stating what it does. For example `it 'includes the source and destination encodings in the return value'` or something along those lines.