= can: improve HttpManager extensibility by making child creation
overridable

Converts HttpManager's instantiations of `HttpClientSettingsGroup` and `HttpListener` to use the virtual class pattern, so it is easily overridable without having to duplicate the rest of the functionality in the class.
Merge pull request #602 from jrudolph/i/601/big-write-problem

= io: fix garbling of big Tcp.Writes, fixes #601
Merge pull request #592 from jrudolph/i/281/chunkless-request-streaming

+ can: implement chunkless request streaming, refs #281
Merge pull request #596 from jrudolph/i/532/allow-retries-if-connection-attempts-fail

= can: also retry non-idempotent requests if connection attempt failed, fixes #532
Merge pull request #597 from jrudolph/w/streamline-future-marshalling

Streamline completing by replacing CompletionMagnet in favor of the more generic ToResponseMarshallable
Merge pull request #595 from jrudolph/i/205/easier-probes

! testkit: move result.awaitResult call from `injectIntoRoute` into `check`, fixes #205
Merge pull request #589 from lihaoyi/master

Virtual classes for HttpManager
= routing: move 'complete with future' tests around
+ httpx: add ToResponseMarshaller.oneOf for negotiated marshalling from several (sub-)marshallers, refs #581
+ httpx: Unmarshaller.oneOf to create negotiating Unmarshaller, fixes #581