Fix: handle single-byte RLP strings, bump to v0.5.14

[samuelarogbonlo]

Fixes the RLP decoder to properly handle single-byte strings and bumps version to 0.5.14. Previously, attempting to decode a single-byte string like "a" would result in the error "RLP string ends with -32 superfluous bytes".

Changes:

• Modified encoder to consistently wrap output in Rlp::Data
• Fixed handling of single-byte strings in RLP encoding/decoding
• Added test case to verify single-byte string handling
• Bumped version from 0.5.13 to 0.5.14

Tests:

• All existing tests pass
• Added new test case for single-byte strings
• Verified fix with direct testing of the failing case:

  Eth::Rlp.decode(Eth::Rlp.encode("a")) # now works correctly