[Feature] Replace underscore in header-name with dash

[leberknecht]

While i'm aware that underscores are totally legal in header-names, a lot of libs / software doesnt play well with it (nginx for example requires to explicitly allow them via underscores_in_headers on; ..for whatever reason. Also symfonys BrowserKit component replaces the underscore with a dash, and there are more examples like that).

While this is not yobulks fault, i still think that, for broader adaption, it would make sense to change the header template_id to template-id (or even Template-Id maybe?) as that is simply more-common.

Opinions?