Support nested usage of JsonApiClient::Resource.with_headers

[tom-lord]

The problem:

When the .with_headers block closes, it wipes any headers that have been set. This can lead to counterintuitive issues in code; consider:

def my_record
  JsonApiClient::Resource.with_headers("Authorization: ...") do
    # fetch data
  end
end

def update_record
  JsonApiClient::Resource.with_headers("Authorization: ...") do
    my_record.save
  end
end

Regardless of whether the headers inside both blocks are the same or different, the JsonApiClient "forgets" them, halfway through the block!

The current workaround might be to instead do:

def my_record
    JsonApiClient::Resource.with_headers("Authorization: ...") do
      # fetch data
    end
end

def update_record
  record = my_record
  JsonApiClient::Resource.with_headers("Authorization: ...") do
    record.save
  end
end

...But this is confusing, and cumbersome.

My proposal in this PR is to "reset" the headers to their previous state at the end of a .with_headers block, rather than wiping hem completely.