Allow passing multiple acceptable content types to accept route selector

[rock3r]

Subsystem
Server, route building

Is your feature request related to a problem? Please describe.
Right now the accept() route selector only takes a single content type.

Describe the solution you'd like
It would be convenient if we could pass more than one, and it could match on any of the ones provided.

Motivation to include to ktor
That would reduce the amount of duplication when multiple content types are handled by the same handler.

[oleg-larshin]

Please check the following ticket on YouTrack for follow-ups to this issue. GitHub issues will be closed in the coming weeks.