External redirects: non-web URLs are treated as servlet-relative paths

[liujed]

External redirects treat non-web URLs as servlet-relative paths. For example,

• mailto:help@mail.test is redirected to /springtravel/app/mailto:help@mail.test,
• tel:+1.800.555.1212 is redirected to /springtravel/app/tel:+1.800.555.1212, and
• my-mobile-app://redirect/target/path is redirected to /springtravel/app/my-mobile-app://redirect/target/path.

This stems from logic in FlowHandlerAdapter that has special cases for http:// and https://, but can be generalized to handle more kinds of URLs.