Strip X-Forwarded auth headers from whitelisted paths (#624)

* Strip X-Forwarded auth headers from whitelisted paths

For any paths that match skip-auth-regex, strip normal
X-Forwarded headers that would be sent based on pass-user-headers
or pass-access-token settings. This prevents malicious injecting
of authentication headers through the skip-auth-regex paths in
cases where the regex might be misconfigured and too open.
Control this behavior with --skip-auth-strip-headers flag. This
flag is set to TRUE by default (this is secure by default, but
potentially breaks some legacy configurations).

Only x-Forwarded headers stripped, left the Authorization header
untouched.

* Strip authorization header if it would be set

* Improve TestStripAuthHeaders test table

* Improve --skip-auth-strip-headers flag documentation