Respect number of bytes returned by value decoders

[soundmonster]

Description

Respect the number of bytes generated by value decoders for hex and Base64.

Fixes issue #227: producing from a file with Base64-encoded values that start with zeros results in empty values in the produced messages. Payloads that start with "AA.." are common for e.g. Avro.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Documentation

• the change is mentioned in the ## [Unreleased] section of CHANGELOG.md
• the configuration yaml was changed and the example config in README.adoc was updated
• a usage example was added to README.adoc
• tests for the changes have been implemented (see: Testing your changes)

[soundmonster]

Sorry for the failed build, I forgot to add the JSON file fixture. I've fixed it now and tested on my fork.