title: Warning message when generating STIX in Python
body: <p>When running a simple example I get a warning I can't find the reason for.</p>
  
  <pre><code>from stix.core import STIXPackage, STIXHeader   # Import the STIX Package and STIX Header APIs
  
  stix_package = STIXPackage()                    # Create an instance of STIXPackage
  stix_header = STIXHeader()                      # Create an instance of STIXHeader
  stix_header.description = "Getting Started!"    # Set the description
  stix_package.stix_header = stix_header          # Link the STIX Head to our STIX Package
  
  print(stix_package.to_xml())                    # print the XML for this STIX Package
  </code></pre>
  
  <p>Results in</p>
  
  <p><strong>C:\Python27\Lib\site-packages\stix\utils\deprecated.py:40: UserWarning: The use of this field has been deprecated. Received 'str' object.
    warnings.warn(msg)</strong></p>
  
  <pre><code>&lt;stix:STIX_Package
          xmlns:cybox="http://cybox.mitre.org/cybox-2"
          xmlns:cyboxCommon="http://cybox.mitre.org/common-2"
          xmlns:cyboxVocabs="http://cybox.mitre.org/default_vocabularies-2"
          xmlns:example="http://example.com"
          xmlns:stix="http://stix.mitre.org/stix-1"
          xmlns:stixCommon="http://stix.mitre.org/common-1"
          xmlns:stixVocabs="http://stix.mitre.org/default_vocabularies-1"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" id="example:Package-7e8da451-3a23-47dd-b2e1-8646200dadf0" version="1.2"&gt;
      &lt;stix:STIX_Header&gt;
          &lt;stix:Description&gt;Getting Started!&lt;/stix:Description&gt;
      &lt;/stix:STIX_Header&gt;
  &lt;/stix:STIX_Package&gt;
  </code></pre>
  
  <p>tips appreciated</p>