Handle "fetch directives" when appending to a policy config

Bug fix for handling policy merges where appending a non-default source value (report-uri, plugin-types, frame-ancestors, base-uri, and form-action) would be combined with the default-src value. Appending a directive that doesn't exist in the current policy combines the new value with default-src to mimic the actual behavior of the addition. However, this does not make sense for non-default-src values (a.k.a. "fetch directives") and can lead to unexpected behavior like a report-uri value of *. Previously, this config:

{
  default_src => %w(*)
}

When appending:

{
  report_uri => %w(https://report-uri.io/asdf)
}

Would result in default-src *; report-uri * which doesn't make any sense at all.