Fix proxy_set_header for default location
Merge pull request #467 from invliD/proxy-header-fix

Impossible to set proxy_set_header for default location
spec: cleanup

* Removed some tests that no longer apply and/or are redundant
* Only set OS facts when testing OS-specific behaviors. This simplifies the
  facts that must be set in the specs and saves running the same tests
  several times when the results wouldn't differ by OS anyway.
package: don't install upstream package on Amazon Linux

Upstream doesn't provide a build of Nginx for Amazon Linux.
package: install "nginx" package when OS isn't recognized

This will provide broader OS compatibility without having to manually add
support for specific OSes.
package: removed a warning for Fedora 18 and below

Those versions no longer supported by the Fedora Project.
package: Differentiate between upstream's RedHat and CentOS packages

Even if they're currently identical they may diverge in future.
package: package resource is titled "nginx"
package/debian: fail when `$package_source` is invalid
package: only set `manage_repo => true` when appropriate

i.e. upstream provides a package for it
package: refactor
package: fail on Solaris when `$package_source` not set

The comments say this must be set on that OS so the failure makes that
explicit.