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

[Julia Boes]

Add two static factory methods to com.sun.net.httpserver 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) {}`

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Commit messages:
 - update specs and expand test
 - initial commit
 - (drive-by) add detail msg for AssertionError

Changes: https://git.openjdk.java.net/jdk/pull/3468/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3468&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265123
  Stats: 282 lines in 3 files changed: 279 ins; 0 del; 3 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