Akka HTTP RequestBuilding should have better configurable timeouts

**Issue by [zifeo](https://github.com/zifeo)**
_Friday Jun 17, 2016 at 21:40 GMT_
_Originally opened as https://github.com/akka/akka/issues/20798_

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Defining a timeout for `RequestBuilding` is quite verbose as overloaded signatures do not provide all implicit variables, per se revolved to 1 second. This is especially an issue when having route (integration) tests depending on async computations.

Missing signature : [RequestBuilding.scala#L28](https://github.com/akka/akka/blob/master/akka-http/src/main/scala/akka/http/scaladsl/client/RequestBuilding.scala#L28)
Complete signature: [RequestBuilding.scala#L43](https://github.com/akka/akka/blob/master/akka-http/src/main/scala/akka/http/scaladsl/client/RequestBuilding.scala#L43)

Start in akka-http/src/main/scala/akka/http/scaladsl/client/RequestBuilding.scala at the linked lines 28 and 43, comparing the existing overloads and their timeout handling. Check the surrounding client API tests and route integration tests; done means a less-verbose configurable timeout signature is available without the current default being forced.

Written by the indexing model from the issue text.