Fix issue.save for new issues

[shortdudey123]

If the issue is new, then the fields are not being updated, they are being created.

new_issue_data = {
  'fields' => {
    'reporter' => {
      'name' => 'username'
    },
    'project' => {
      'key' => 'KEY'
    },
    'issuetype' => {
      'name' => 'Task'
    },
    'summary' => 'Testing ticket',
    'description' => 'blah blah blah'
  }
}

new_issue = Jiralicious::Issue.new(new_issue_data)
saved_issue = new_issue.save!

The above code results in 400 from Jira due to the fields hash being empty

Jiralicious::TransitionError: #<HTTParty::Response:0x7ffd32d57730 parsed_response={"errorMessages"=>[], "errors"=>{"project"=>"project is required"}}, @response=#<Net::HTTPBadRequest 400 Bad Request readbody=true>, ...

Options hash from just before the API call in Jiralicious::Base#fetch

{:method=>:post, :body=>{"fields"=>{}}, :body_uri=>{"fields"=>{}}, :url_uri=>"https://example.atlassian.net/rest/api/latest/issue/"}