Fix crash while streaming

Found using fuzzing. Reproduction with pathoc, given "mitmproxy -s" and
pathod running on 9999:

get:'http://localhost:9999/p/':s'200:b\'foo\':h\'Content-Length\'=\'3\'':i58,'\x1a':r

 return flow.FlowMaster.run(self)
   File "/Users/aldo/mitmproxy/mitmproxy/libmproxy/controller.py", line 111, in run
     self.tick(self.masterq, 0.01)
   File "/Users/aldo/mitmproxy/mitmproxy/libmproxy/flow.py", line 613, in tick
     return controller.Master.tick(self, q, timeout)
   File "/Users/aldo/mitmproxy/mitmproxy/libmproxy/controller.py", line 101, in tick
     self.handle(*msg)
   File "/Users/aldo/mitmproxy/mitmproxy/libmproxy/controller.py", line 118, in handle
     m(obj)
   File "/Users/aldo/mitmproxy/mitmproxy/libmproxy/flow.py", line 738, in handle_responseheaders
     self.stream_large_bodies.run(f, False)
   File "/Users/aldo/mitmproxy/mitmproxy/libmproxy/flow.py", line 155, in run
     r.headers, is_request, flow.request.method, code
   File "/Users/aldo/mitmproxy/mitmproxy/netlib/http.py", line 401, in expected_http_body_size
     raise HttpError(400 if is_request else 502, "Invalid content-length header: %s" % headers["content-length"])
 netlib.http.HttpError: Invalid content-length header: ['\x1a3']