change CodedOutputStream.buffer type to Box<[u8]>
change CodedOutputStream.buffer type to Box<[u8]>

... because previously len was always equal to capacity.

danburkert/rust-protobuf,stepancheg/rust-protobuf,stepancheg/rust-protobuf,danburkert/rust-protobuf,stepancheg/rust-protobuf,danburkert/rust-protobuf