Spawn::Child should support encodings properly under 1.9

[rtomayko]

Right now we're forcing the encoding of the input string to BINARY and also setting the output streams to BINARY for reads. See:

af16f2a

To be consistent with Ruby's core IO methods we'd need to.

1. Accept input in any encoding and transcode to Encoding::default_external or a user-specified external encoding when writing to the stdin fd.
2. Read data off of the stdout/stderr assuming the default Encoding::default_external or user-specified external_encoding and transcode to the Encoding::internal_encoding.
3. Add support for overriding the default external/internal encodings as options to Child::new.

One weird thing I noticed when working on this previously is that IO#readpartial and IO#write_nonblock seem to ignore the IO object's encoding, unlike the IO#read and IO#write methods. We get BINARY encoded data out no matter what the external/internal encoding is set to. Something to look into when we fix this.