update_oai-pmh Improve HTTP request headers

Motivation:

The User-Agent request header was not being sent due to a bug.

The havester lacks support for the From request header.

Both HTTP request headers are recommented by the Harvester's guidelines:

https://www.openarchives.org/OAI/2.0/guidelines-harvester.htm

Modification:

Fix bug with User-Agent header. Add support for From header.

The From header takes an email address as an argument. Rather than requiring a configuration file, the code looks for likely places where the user may have already configured their email address.

Result:

The OAI-PMH harvester more closely followes the corresponding guidelines.