eth/util: fix single-byte hex-string nibbles

[q9f]

fix single-byte hex-string nibbles:

[1] pry(main)> Util.hex_to_bin "a"
=> "\n"
[2] pry(main)> Util.bin_to_hex "\n"
=> "0a"
[3] pry(main)> Util.bin_to_hex Util.hex_to_bin "a"
=> "0a"

this fixes the bug raised in #292

[1] pry(main)> Rlp.encode "a"
=> "a"
[2] pry(main)> Rlp.decode "a"
=> "\n"
[3] pry(main)> Rlp.decode Rlp.encode "a"
=> "\n"
[4] pry(main)> Util.bin_to_hex Rlp.decode Rlp.encode "a"
=> "0a"