Use the proper HTTP method when saving a repository

The previous implementation of `Repository.save` was destructive and had
the nasty side-effect of wiping all of a repository’s content when it’s
configuration was updated! For more on this WTF see:

http://www.jfrog.com/confluence/display/RTF/Artifactory+REST+API#ArtifactoryRESTAPI-CreateorReplaceRepositoryConfiguration