Handling empty header values

Currently, we ignore empty header values and consequently when we serialize it we will lose that information. I don't know what the RFC says a proxy should do, but I think it's safe to pass on whatever the client sent us in it's entirety. Also, this is polluting the logs with error messages such as

20130611.17h48m22s [atscppapi] [src/Headers.cc:119, checkAndInitHeaders()] Failed to get value at index 0 of header [Accept-Encoding]

In this patch we retain the empty header values ensure we pass it along to the origin and also avoid the whole error logging thing.