title: in JIRA python API, how to fetch certain fields from the issue?
body: <p>I'm using python API for JIRA and from a few days, facing a problem while fetching the particular field types from issues. I'm using below code and it seemed to work fine till few days ago, but now it throws me an error:
  The code I'm using is:
      issues = jiraDC.search_issues('project={}'.format(project_id),  start,size, fields=['issuetype', 'project', 'status', 'priority', 'created', 'customfield_14729', 'customfield_14731','customfield_14802'])</p>
  
  <p>The error I'm getting on running this code is:
      File "", line 32, in 
    File "/usr/lib/python2.7/site-packages/jira/client.py", line 2084, in search_issues
      fields = fields.copy()
  AttributeError: 'list' object has no attribute 'copy'</p>
  
  <p>When I'm fetching all the fields, then there is no error coming, but this is slowing my code by a great deal. 
  Can somebody point me in the right direction?
  Thanks</p>