Fixing --http-header option

[michaelschem]

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Link to redmine ticket:
https://redmine.openinfosecfoundation.org/issues/3696

Describe changes:

• In the main.py file, http_header was incorrectly being used as a key to get the http-header value from sources. The key was updated to http-header to fix this.

[michaelschem]

The option --http-header documented here can set an additional header. It is saved in the /var/lib/suricata/update/sources directory in a yaml, but main.py isn't able to be retrieved it due to using the an underscore (_) instead of a hyphen (-) in the key http-header.

[jasonish]

Merged. Thanks!