fix use of deprecated code in HttpEntity

[pjfanning]

[warn] /Users/pj.fanning/code/incubator-pekko-http/http-core/src/main/scala/org/apache/pekko/http/scaladsl/model/HttpEntity.scala:666:23: method getFirst in class Attributes is deprecated (since Akka 2.5.7): Attributes should always be most specific, use get[T]
[warn]           _attributes.getFirst[SizeLimit] match {

see akka/akka-http#3463

We might need to undeprecate getFirst.

See #346 - using get instead getFirst breaks lots of tests.

wdyt @jrudolph @mdedetrich @raboof ?

[jrudolph]

Yep, we tried that before and did not find an easy solution. In the best case, we would dig in and understand what the differences are and understand how to fix the problems in Pekko HTTP.