Bugfix
- [ #107 ] ignored the UTF-16 surrogate pair after an invalid unicode
- [ #108 ] couldn't skip zero-based exponent in skip_number()
- [ #112 ] returned error when unmarshalling invalid base64-encoded strings
- [ #113 ] redundantly handled struct-typed fields' "omitempty" tag
- [ #115 ] encoder.SortMapKeys flag didn't work when marshalling integer-key map