String#encode should not convert binary to mangled UTF-8

In Moped (my MongoDB driver), I have some code attempting to safely convert input text into UTF-8 (which all strings must be).

See for a simplified test-case of the implementation: https://gist.github.com/2351047

Problem: that calling encode('utf-8') on a binary string creates garbage utf-8 text instead of raising a conversion error like 1.9.

Reason the code exists: We're tracking referrers in our app, but the header value in the rack environment is encoded as ASCII-8bit; this code then follows the happy-path of calling `encode('utf-8')` and finally tries forcing the encoding when that raises an error.

Tested on: jruby-1.6.7 and jruby-1.7.0-dev