title: Does requests properly support multipart responses?
body: <p>I'm getting an error when receiving a multipart response.</p>
  
  <pre><code>WARNING connectionpool  Failed to parse headers (url=************): [StartBoundaryNotFoundDefect(), MultipartInvariantViolationDefect()], unparsed data: ''
  Traceback (most recent call last):
    File "/usr/local/lib/python3.6/site-packages/urllib3/connectionpool.py", line 399, in _make_request
      assert_header_parsing(httplib_response.msg)
    File "/usr/local/lib/python3.6/site-packages/urllib3/util/response.py", line 66, in assert_header_parsing
      raise HeaderParsingError(defects=defects, unparsed_data=unparsed_data)
  urllib3.exceptions.HeaderParsingError: [StartBoundaryNotFoundDefect(), MultipartInvariantViolationDefect()], unparsed data: ''
  </code></pre>
  
  <p>Does this mean that the library does not support multipart responses?  The response from my server works in all other cases including to the browser so I'm a little confused.</p>
  
  <p>Any ideas?</p>
  
  <p>This is what is coming back from the server (of course body truncated for brevity):</p>
  
  <pre><code>HTTP/1.1 200 OK
  X-Powered-By: Servlet/3.1
  X-CA-Affinity: 2411441258
  Cache-Control: no-cache
  Expires: Thu, 01 Jan 1970 00:00:00 GMT
  Content-Encoding: gzip
  X-Compressed-By: BICompressionFilter
  Content-Type: multipart/related; type="text/xml"; boundary="1521336443366.-7832488688540884419.-1425166373"
  Content-Language: en-US
  Transfer-Encoding: chunked
  Date: Sun, 18 Mar 2018 01:27:23 GMT
  
  a
  
  154e
  &lt;i  ʲ O  x\龅L   dre     Qyi  
  /su  k 
  </code></pre>
  
  <p>Of course this is encoded.  If I decode it in Fiddler this is what it looks like:</p>
  
  <pre><code>HTTP/1.1 200 OK
  X-Powered-By: Servlet/3.1
  X-CA-Affinity: 2411441258
  Cache-Control: no-cache
  Expires: Thu, 01 Jan 1970 00:00:00 GMT
  X-Compressed-By: BICompressionFilter
  Content-Type: multipart/related; type="text/xml"; boundary="1521336443366.-7832488688540884419.-1425166373"
  Content-Language: en-US
  Date: Sun, 18 Mar 2018 01:27:23 GMT
  Content-Length: 17419
  
  
  --1521336443366.-7832488688540884419.-1425166373
  Content-Type: text/xml; charset=utf-8
  Content-Length: 15261
  
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  </code></pre>