performance improvement for ResteasyHttpHeaders

Using a static cache of standard language Locales and MediaTypes speeds up calls to getAcceptableMediaTypes() and getAcceptableLanguages()

• when there is no such header, or
• there is a header but it contains only a wildcard, or
• there is a header which contains only one entry which is one of the standard MediaTypes or language Locales.

This will not change behavior when there is a non-standard language Locale or MediaType, or if there are multiples.

A copy of the modified class is attached - changes are marked with 
      // Liberty change
flags, as we have made these changes in the overlay we use in WebSphere Liberty.