Prior to Puppet Server 5, Puppet Server's http client class had an
implementation of `get` with an arity of 2 (where `path` and `headers` are the
arguments). However, in Ruby Puppet and in Puppet Server 5, the http client
class has an implementation of `get` with an arity of 3 (with arguments
`path`, `headers`, and `options`).
Since we do not know under which version of Puppet Server or the Ruby Puppet
master this terminus code will be run under, this commit adds a
`multi_arity_get` helper function that inspects the `http_instance` class to
determine the arity of `get` before calling it with the appropriate arguments
for its arity.