Fix four HTTP proxying problems.
 * Added "Connection: close" to 5xx errors.
 * Properly close both upstream and downstream connections on errors.
 * Fixed infinite loop when responding to HEAD requests that have no body.
 * Fixed randomness when using keep-alive connections. Previously, the behavior
   was only randomly selected on new connections, but HTTP keep-alive connections
   need to choose a random behavior for each new request/response.
Merge pull request #22 from jterrace/fix-http2

Fix four HTTP proxying problems.