Adding changeOrigin for apiProxy

changeOrigin instructs the HTTP proxy that it should rewrite the host
header to match the target hostname. For example, if
server.apiProxy.host = 'foo.dev':

changeOrigin = false would send "Host: localhost:8000" to the proxied
API (current behaviour, still the default)

changeOrigin = true would send "Host: foo.dev" to the proxied
API (desirable if the target host does host-based lookups, like Apache
VirtualHost)