Fix "Malformed UTF-8" problem on MoarVM

Mokudo encounters an interesting error in the test suite: it complains of malformed UTF-8 buffers.  MoarVM treats `"\x[c7,82]".encode('latin-1')` (for example) differently than Parrot or the JVM, and I'm not sure whether what it is doing is wrong.  So here's a fix that makes the module work across all three backends.