bug: proxy headers aren't forwarded properly

The proxy currently forwards headers in WSGI format,
e.g. 'Content_Length', vs. 'Content-Length'. This breaks any standards
conforming HTTP server that's placed in between the proxy and
marconi-queues endpoints (for partitioning). This patch cannonicalizes
headers before they are forwarded to resolve this issue.

Change-Id: I1eab83dbb1fb9cfec5d15d40830e383da4e6591e