Update ProxyServlet.java

Fixes issue https://github.com/mitre/HTTP-Proxy-Servlet/issues/41
Preserves cookie path

Original path of cookie is preserved by appending it to the servlet path.
Preserves HttpOnly

Cookie from the proxy preserves has the HttpOnly property set if the original cookie had it also set.
Fix for older apis
Merge pull request #42 from chw-1/patch-1

Fix #41 Copied cookie path may be invalid