This updates REST Docs to test against Framework's latest 5.1
snapshot. Due to a bug in the behaviour of HttpHeaders' key set [1],
HeaderRemovingOperationPreprocessor has been updated to no longer use
it when removing headers from a request or response prior to it being
documented.
[1] https://github.com/spring-projects/spring-framework/issues/22821
Previously, MockHttpServletRequest allowed its body to be ready
multiple times using its InputStream. As of recent Spring Framework
5.1 snapshots, this is no longer the case.
This commit moves to directly accessing the request's content as a
byte array using getContentAsByteArray() which was introduced in
Framework 5.0.
Closes gh-515
