Fix an indexing panic in compression (#578)

This guard was backwards, not only breaking the functionality it was
intended to provide, but also causing panics under certain conditions.

In this case, the haystack is the pre-existing Vary header on the
response, and the needle is "Accept-Econding".

If we are compressing a response, and there is already a Vary header on
the response, it is <= 15 bytes and does not match Accept-Encoding, this
would cause a range index error as the while statement would not provide
an effective guard.

Author: sulami

tower-http/src/compression/future.rs

@@ -122,7 +122,7 @@ where
 }
 
 fn contains_ignore_ascii_case(mut haystack: &[u8], needle: &[u8]) -> bool {
-    while haystack.len() <= needle.len() {
+    while needle.len() <= haystack.len() {
         if haystack[..needle.len()].eq_ignore_ascii_case(needle) {
             return true;
         }