Upgraded fluent-hc to 4.3.4 due to bug

There seems to be a bug introduced by merging the last pull request that moved to `fluent-hc`. Submitting my fix.

This bug (https://issues.apache.org/jira/browse/HTTPCLIENT-1474) causes
an error to occur when POSTing to datadog using fluent static
Request.Post() method.
Removed unnecessary HttpPost field from HttpTransport