title: Script Runner: handle custom field with space sin expressions
body: <p>I have a custom User Picker (single user) field. This field has a space in the name (e.g. "User Picker"). I cannot remove the space from this name as it is used in searches by many other people.</p>
  
  <p>I want to search for all the issues where the <code>assignee != User Picker</code>. I installed Script Runner and added this in the advanced search:</p>
  
  <pre><code>issuefunction in expression("", "assignee != User Picker")
  </code></pre>
  
  <p>I got the error:</p>
  
  <pre><code>expecting EOF, found 'Picker' @ line 1, column 25.
  </code></pre>
  
  <p>I then searched the net and found that I should be able to use a custom field ID. For example, for <code>cf[12345]</code> I should be able to use:</p>
  
  <pre><code>issuefunction in expression("", "assignee != customfield_12345")
  </code></pre>
  
  <p>This results in the error:</p>
  
  <pre><code>Field customfield_12345 not found or not a Number or Date custom field.
  </code></pre>
  
  <p>Is there a way of using User Picker custom fields with spaces in expressions?</p>