add Headers.set_raw, remove unsafe from .get_raw

Internal representation was changed from an enum back to a Struct again.
The raw representation *has* to stick around, even if parsed as a proper
typed header. The reason is that internally, hyper will access some
headers to know which parts of the http protocol to follow (such as if
the response has a length or is chunked). The raw value may still be
needed afterwards, such as for a DOM binding of
.getAllResponseHeaders().

Since the raw is kept around, the unsafety of get_raw is no longer true,
and so that is removed.

It's still more ergonomic to access the types, and safer as well, so
that is recommended when possible.
Merge pull request #70 from hyperium/raw

add Headers.set_raw, remove unsafe from .get_raw