ForwardedHeader should accept multiple elements

[schabe77]

Hello,

https://tools.ietf.org/html/rfc7239 states that it's possible to have more than one element in the "Forwarded"-Header.
Each elements can consist of multiple field-values, separated by semicolon.
Elements are seperated by comma (see examples at https://tools.ietf.org/html/rfc7239#section-4).

But org.springframework.hateoas.mvc.ForwardedHeader.of(String) only splits by semicolon.
That's why parsing Headers like
ForwardedHeader.of("for=192.0.2.43, for=198.51.100.17");
fails with
java.lang.IllegalArgumentException: At least one forwarded element needs to be present!
at org.springframework.util.Assert.isTrue(Assert.java:116)
at org.springframework.hateoas.mvc.ForwardedHeader.of(ForwardedHeader.java:68)

Multiple Forward-Elements are used by opera mini, for example. Opera users may use opera servers as proxy to reduce data usage. These Proxies add a Forward-Header with multiple Values.

[mle-enso]

I can confirm this behaviour:

This one works (in conjunction with a dummy controller at path "/dummy"):

    @Test
    public void buildUrlWithOneForInForwardedHeader() {
        // given
        MockHttpServletRequest request = new MockHttpServletRequest();
        request.addHeader("Forwarded", "for=111.0.0.0; by=host=example.com");
        RequestContextHolder.setRequestAttributes(new ServletRequestAttributes(request));

        // when
        String url = linkTo(methodOn(DummyController.class).getDummy()).withSelfRel().getHref();

        // then
        assertThat(url).isEqualTo("http://example.com/dummy");
    }

…and this one doesn't (throws the exception mentioned above)…

    @Test
    public void buildUrlWithTwoForInForwardedHeader() {
        // given
        MockHttpServletRequest request = new MockHttpServletRequest();
        request.addHeader("Forwarded", "for=111.0.0.0, for=222.0.0.0; by=host=example.com");
        RequestContextHolder.setRequestAttributes(new ServletRequestAttributes(request));

        // when
        String url = linkTo(methodOn(DummyController.class).getDummy()).withSelfRel().getHref();

        // then
        assertThat(url).isEqualTo("http://example.com/dummy");
    }

[gregturn]

I appreciate you investigating this along with finding the spec reference. I'll keep that in mind as we look to properly integrate with Spring MVC's updates in 5.1 regarding Forwarded header handling. It's quite possible we will be able to delegate completely, making this a moot point. We'll see.

[gregturn]

In light of #713, I'm closing this issue.