Fix string encoding support for Ruby 1.9

Submitted by Will Benton <willbenton@gmail.com>

This patch ensures that all returned strings are treated as if they were of the default external encoding in Ruby 1.9. If we don't do this, then strings returned from QMF (e.g. as arguments) will have a nominal encoding of 'ASCII-8BIT' (i.e. raw bytes) and will not be comparable to strings created from within Ruby code or loaded from other sources. (Note that the ensure_encoding method could take a user-specified encoding; if it becomes necessary in the future to allow QMF users to specify encodings, this would be a straightforward change.)