InvokeHTTP support for sending a Useragent

[Niels Basjes <Ni...@basjes.nl>]

Hi,

I recently found that the InvokeHTTP processor does not send a User-agent
header with the request.

To solve this I filed issue NIFI-6666 (yes, really that number ...  on
Friday the 13th ... ) and I put up a pull/merge request with a proposed fix.

https://issues.apache.org/jira/browse/NIFI-6666

https://github.com/apache/nifi/pull/3734

I have two requests:
1) Can you guys please make me a contirbuter of Nifi (username nielsbasjes)
so this ticket can correctly be assigned to me.
2) I would love to get feedback on how I solved this. Main question: Did I
do it right in light of how Nifi works.

To get sensible data in the useragent I made some build time variables
available in the expression language and then having an expression with a
good looking useragent string was easy.

In addition the question is if you agree with the default useragent I came
up with (see pull request for details).

-- 
Best regards / Met vriendelijke groeten,

Niels Basjes