handle http headers when behind a well configured proxy

Fixes Unidata/thredds#1310

This should work with the following Nginx proxy config:

```
    location /thredds/ {
        proxy_pass http:https://thredds:8080/thredds/;
        proxy_set_header Host $host;  # pass the original public hostname to Thredds
        proxy_set_header X-Forwarded-Proto $scheme;  # pass the original httpS proto to Thredds
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;  # pass the original client IP to Thredds
    }
```

server.xml

@@ -128,6 +128,19 @@
  <Engine name="Catalina" defaultHost="localhost" jvmRoute="jvm1">
  -->
  <Engine name="Catalina" defaultHost="localhost">
+ <!--
+ Handle X-Forwarded-Proto and X-Forwarded-For http header when
+ behind a well configured proxy.
+
+ * X-Forwarded-Proto is to preserve the original proto (httpS)
+ from the client connecting URL.
+
+ * X-Forwarded-For (set by default by the Valve) is to preserve the
+ original client IP in the logs. Otherwise the nearest proxy IP
+ will be logged, much less useful for auditing.
+ -->
+ <Valve className="org.apache.catalina.valves.RemoteIpValve"
+ protocolHeader="X-Forwarded-Proto" />
 
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