改进内容：
** Bug
* [MNG-3265] - maven-model Extension.equals causes NPE when any field is uninitialized
* [MNG-3506] - Custom ArtifactHandler not resolved for project when an
additional plugin with extensions is defined in parent pom
* [MNG-3753] - ArtifactResolverDiagnoser.diagnose() fails with NPE if nested IOException has no detail message
* [MNG-4189] - Maven not picking up specific timestamped version
dependency when a later timestamped version was downloaded and already
present in the local repository
* [MNG-4218] - NPE in AbstractArtifactResolutionException if DefaultArtifactResolver.resolveTransitively is interrupted
* [MNG-4228] - [regression] Authorization failed: Not authorized by proxy.
* [MNG-4235] - [regression] Maven 2.2.0 produces invalid checksums during deployment to secured HTTP repo
* [MNG-4236] - [regression] http wagon uploads files twice with Maven 2.2.0 when preemptive auth is disabled (default setting)
* [MNG-4238] - Custom ArtifactHandler provided by build extension isn't used for project artifact
* [MNG-4240] - Direct dependencies with scope == provided will not have
their transitive dependencies resolved for compiling and testing
* [MNG-4270] - ArtifactHandler, LifecycleMapping from plugin dependency is not used when plugin extensions are enabled
* [MNG-4275] - [regression] Direct relocations no longer log at WARNING level : MNG-3380 conflicts with MNG-1689
** Improvement
* [MNG-4254] - Support selection of wagon implementation for a particular protocol
* [MNG-4279] - wagon provider selection should fail gracefully and use
protocol for roleHint if protocol-provider roleHint isn't available.
** Task
* [MNG-4290] - Update guide-http-settings to reflect the fact that
sun-based http has been restored as the default for the http/s wagons.
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