Replace getFirst with get

[mdedetrich]

So getFirst for getting an attribute is deprecated and you are instead meant to use get however this does have a different behaviour change (i.e. getFirst will get the first found attribute where as get will get the most specific attribute).

Since we are now in the 1.1.x branch these kinds of behaviour changes are allowed

[mdedetrich]

Okay so as I expected, we are actually relying on the behaviour of getFirst since the tests have started failing, will do some digging here.

[mdedetrich]

@jrudolph @raboof So I had a quick look at this and I don't know what to do here, it seems like making getFirst deprecated was a mistake or maybe it should only be deprecated for public and not private inter-pekko use cases?

The whole reason why getFirst is necessary is so that the withSizeLimit directive works correctly with nested routes and while it is possible to remove the deprecation of getFirst without using get in doing so it would just involve copying the implementation of it which means its not really solving a problem?