Allow disabling proxy_http_version directive

The EPEL 6 repository still has version 1.0.15 which does not support
this directive proxy_http_version. Yes, this version is old, but a lot
of people prefer not to use a bleeding edge version from nginx.org.
Disable proxy_http_version directive by default

The default value in nginx::config was the same as nginx's default,
so it's not necessary to be set in any version of nginx unless one
needs a specific value. This also allows us to use undef when we don't
want the directive to be set instead of false.

Also added a test for the default case.
Merge pull request #531 from ckaenzig/optional-proxy-http-version

Allow disabling proxy_http_version directive