RFR: 8265123: Add static factory methods to com.sun.net.httpserver.Filter [v6]

[Julia Boes]

> Add two static factory methods to com.sun.net.httpserver.Filter that facilitate the creation of pre- and post-processing Filters: 
> 
> `public static Filter beforeResponse(String description, Consumer<HttpExchange> filterImpl) {}`
> `public static Filter afterResponse(String description, Consumer<HttpExchange> filterImpl) {}`

Julia Boes has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains ten additional commits since the last revision:

 - update @return description
 - Merge branch 'master' into 8265123
 - some more spec updates and small test cleanup
 - update specs and small fix in test
 - fix empty response body
 - update specs and add test
 - update specs and expand test
 - initial commit
 - (drive-by) add detail msg for AssertionError

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3468/files
  - new: https://git.openjdk.java.net/jdk/pull/3468/files/0e76acd4..cba4b885

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3468&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3468&range=04-05

  Stats: 563626 lines in 5254 files changed: 44584 ins; 511080 del; 7962 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3468.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3468/head:pull/3468

PR: https://git.openjdk.java.net/jdk/pull/3468