Fix headers provided by HeaderConverter interface to http connector. …

[oBJIADo]

I have tried to use debezium server with http-sink-connecter and i have faced 2 issues:

1. When headers are converted using the HeaderConverter interface (e.g., SimpleHeaderConverter), an error will always occur. This is because the converter, as defined by the interface, consistently returns a byte[], whereas the HttpChangeConsumer only works with String headers.
   example debezium props:

...
debezium.sink.type=http
...
debezium.format.header=clientprovided
debezium.format.header.class=org.apache.kafka.connect.storage.SimpleHeaderConverter
...

2. When the endpoint of the server is not reachable then debezium throws npe, because the message is null.

[jpechane]

@oBJIADo Hi, thanks for the PR! Could you please create a Jira issue describing the bug and then modify the commit message to DBZ-xxxx Fix headers provided by HeaderConverter interface to...?
We want to make sure it is correctly tracked. Also that way it will be backported to 3.0 branch too.