fix compilation error

[joseluis]

   Compiling sv v0.2.1 (../rust-sv)
error[E0283]: type annotations needed
   --> src/messages/message.rs:366:83
    |
366 |             Message::Other(s) => Err(io::Error::new(io::ErrorKind::InvalidData, s.as_ref())),
    |                                                                                 --^^^^^^--
    |                                                                                 | |
    |                                                                                 | cannot infer type for type parameter `T` declared on the trait `AsRef`
    |                                                                                 this method call resolves to `&T`
    |
    = note: cannot satisfy `String: AsRef<_>`
help: use the fully qualified path for the potential candidates
    |
366 |             Message::Other(s) => Err(io::Error::new(io::ErrorKind::InvalidData, <String as AsRef<OsStr>>::as_ref(s))),
    |                                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
366 |             Message::Other(s) => Err(io::Error::new(io::ErrorKind::InvalidData, <String as AsRef<Path>>::as_ref(s))),
    |                                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
366 |             Message::Other(s) => Err(io::Error::new(io::ErrorKind::InvalidData, <String as AsRef<[u8]>>::as_ref(s))),
    |                                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
366 |             Message::Other(s) => Err(io::Error::new(io::ErrorKind::InvalidData, <String as AsRef<str>>::as_ref(s))),
    |                                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0283`.
error: could not compile `sv

[joseluis]

PR #7 should be merged, it already fixes this