Issue: SPR-16898

Includes Kotlin 1.2.50, RxJava 2.1.15 and Protobuf 3.6.

Issue: SPR-16475

Issue: SPR-16898

In SPR-16892, the `EncoderHttpMessageWriter` has been improved to write
`"Content-Length"` HTTP response headers if the response body is of type
`Mono` (i.e. the actual content length is easily accessible without
buffering a possibly large response body). That change was relying on
the fact that the server side is using a `ChannelSendOperator` to delay
the writing of the body until the first signal is received.

This strategy is not effective on the client side, since no such channel
operator is used for `WebClient`. This commit improves
`EncoderHttpMessageWriter` and delays, for `Mono` HTTP message bodies
only, the writing of the body so that we can write the
`"Content-Length"` header information once we've got the body resolved.

Issue: SPR-16949

* pr/1860:
  Fix broken link to CONTRIBUTING.md

Closes gh-1860

Issue: SPR-16898

Issue: SPR-16947

The wording changed from "Can not find" to "Cannot find" via PR #1682

Issues: SPR-16947

Issue: SPR-12458

Issue: SPR-12458

Issue: SPR-16833

Issue: SPR-16838

Issue: SPR-12458

Issue: SPR-12458

Allow the `Environment` to accept a generic `Profiles` interface which
can support more complex matching rules. The previous
`acceptsProfiles(String...)` method now uses `Profiles.of` which
supports basic profile expressions such as "(a | b) & !c"

Issue: SPR-12458

Issue: SPR-16336

Related to (but not required by) SPR-16336.

Issue: SPR-16940

This commit adds support for the "SameSite" attribute in response
cookies. As explained in rfc6265bis, this attribute can be used to limit
the scope of a cookie so that it can't be attached to a request unless
it is sent from the "same-site".

This feature is currently supported by Google Chrome and Firefox, other
browsers will ignore this attribute.

This feature can help prevent CSRF attacks; this is why this commit adds
this attribute by default for SESSION Cookies in WebFlux.

See: https://tools.ietf.org/html/draft-ietf-httpbis-rfc6265bis

Issue: SPR-16418

Also, ZeroCopyHttpOutputMessage provides writeWith(Path, int, int), enforcing that variant as the implementation target in 5.1 (analogous to FilePart).

Issue: SPR-16925

Issue: SPR-16889

Issue: SPR-16855

Issue: SPR-16505
Issue: SPR-16499

Issue: SPR-16834
Issue: SPR-16812

To avoid compiler issues on Eclipse.

Issue: SPR-16336