How to set up Scrapy middleware with Proxymesh (or any other proxy service really...)

proxymesh_middleware.py

class ProxyMeshMiddleware(object):

    def process_request(self, request, spider):
        endpoint = os.environ.get('PROXY_ENDPOINT')
        user = os.environ.get('PROXY_USER')
        pw = os.environ.get('PROXY_PW')

        if 'proxy' in request.meta:
            return

        request.meta['proxy'] = endpoint
        proxy_user_pass = '{}:{}'.format(user, pw)
        encoded_user_pass = base64.encodestring(proxy_user_pass)
        request.headers['Proxy-Authorization'] = 'Basic ' + encoded_user_pass