RFR: 8276559: (httpclient) Consider adding an HttpRequest.Builder.HEAD method to build a HEAD request. [v2]

[Jaikiran Pai]

> Can I please get a review for this change which  implements the enhancement noted in https://bugs.openjdk.java.net/browse/JDK-8276559?
> 
> The commit in this PR introduces a new `HEAD()` method on the `HttpRequest.Builder` interface. Given that this is a public interface on a public class of a public module, I decided to add this new method as a `default` method to prevent any potential breakages of any non-JDK implementations of this interface. The internal `jdk.internal.net.http.HttpRequestBuilderImpl` which implements this interface, overrides this default implementation to use an implementation of its own.
> 
> Existing tests have been updated to include coverage for this new method.
> 
> P.S: Slightly unrelated question - is it intentional that the `Builder` interface specifies the visibility modifiers on the interface methods. For example:  `public abstract Builder timeout(Duration duration);`, `public Builder headers(String... headers);` and so on?

Jaikiran Pai has updated the pull request incrementally with four additional commits since the last revision:

 - Implement Daniel's review suggestion - use a different URI for HEAD() request in HttpRequestNewBuilderTest
 - Remove unused imports and fields from HeadTest
 - - Update the HeadTest to use the convenience request builder methods GET() and HEAD()
   - Also add @bug tag to updated tests
 - fix comment about special casing certain HTTP methods

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6348/files
  - new: https://git.openjdk.java.net/jdk/pull/6348/files/4ac69024..f391b869

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6348&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6348&range=00-01

  Stats: 63 lines in 5 files changed: 22 ins; 35 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6348.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6348/head:pull/6348

PR: https://git.openjdk.java.net/jdk/pull/6348