Use JIRA default value for maxResults

If not set, JIRA uses 50 by default.
Fix search iterator

It was all wrong. I could understand it from this example page:

  https://developer.atlassian.com/display/JIRADEV/JIRA+REST+API+Example+-+Query+issues

Also, now we defer the first POST request until the first invocation of
the next_issue method. The set_search_iterator method doesn't POST
anymore.