title: How do I pass multiple StringIO into python-pdfkit?
body: <p>Goal: 
  - Use django templateing language.
  - Render the template in memory (no disk writes).
  - Push rendered content to StringIO instance.
  - Use instance in python-pdfkit.</p>
  
  <p>Issue:
  I keep getting <code>TypeError: coercing to Unicode: need string or buffer, instance found</code> when trying to pass more than one file in the list.</p>
  
  <p>The below code works without the <code>[]</code> and just one StringIO instance. </p>
  
  <p>from django.template import loader, Context
  from django import template
  import StringIO</p>
  
  <pre><code>STATIC_URL = "https://d1i1yohwujljp9.cloudfront.net/static/"
  t = loader.get_template('pdf_coverpage.html')
  c = template.Context( {'STATIC_URL': STATIC_URL })
  output = StringIO.StringIO()
  output.write(t.render(c))
  output1 = StringIO.StringIO()
  output1.write(t.render(c))
  
  pdfkit.from_file([ output, output1 ] , 'out.pdf' )
  </code></pre>
  
  <p>Traceback.</p>
  
  <pre><code>Traceback (most recent call last):
    File "&lt;console&gt;", line 1, in &lt;module&gt;
    File "C:\Python27\lib\site-packages\pdfkit\api.py", line 44, in from_file
      configuration=configuration)
    File "C:\Python27\lib\site-packages\pdfkit\pdfkit.py", line 37, in __init__
      self.source = Source(url_or_file, type_)
    File "C:\Python27\lib\site-packages\pdfkit\source.py", line 12, in __init__
      self.checkFiles()
    File "C:\Python27\lib\site-packages\pdfkit\source.py", line 28, in checkFiles
      if not os.path.exists(path):
    File "C:\Python27\lib\genericpath.py", line 18, in exists
      os.stat(path)
  TypeError: coercing to Unicode: need string or buffer, instance found
  </code></pre>