Set agent http header to specific value

[mhogeweg]

Some sites are blocking requests from http clients with the generic agent header value 'apache' or such. To make requests from Geoportal Server more recognizable, we should set the header to an identified value. while no guarantee on not being blocked or uniqueness or such, it gives some recognizable source of the requests.

this plays particularly for harvesting and federated search.

[pandzel-zz]

Implemented in 1.2.7