Nit comments about other parts of the OpenJDK 9 HTTP Client API details.

HttpHeaders is a pure value type - should be easy to implement (e.g. for use in test
data). Suggestions:

  - The three methods other than map() should have default implementations
    on top of map().

  - (Unsure) Potentially, HttpHeaders could even be a final concrete class.

  - HttpHeaders should override toString()


This issue has been filed to capture comments raised by tobiast at google dot com, see
http://mail.openjdk.java.net/pipermail/net-dev/2017-July/010872.html