Add support for local request headers

[kcdragon]

Allows you to pass in headers to the Savon::Client#call method in the same way as global headers. These headers will be sent with the request.

client.call(endpoint, headers: { 'foo' => 'bar' }) do
  ...
end

[tjarratt]

Thanks for submitting this pull request @kcdragon! I appreciate you taking the time to improve Savon.