Fix overwriting of appended request headers (#493)

* don't overwrite appended user headers

* Fixes tests, all header ordering related

* does not need to clone default headers, added a test

tests/gzip.rs

@@ -113,8 +113,8 @@ fn test_accept_header_is_not_changed_if_set() {
     let server = server! {
         request: b"\
             GET /accept HTTP/1.1\r\n\
-            user-agent: $USERAGENT\r\n\
             accept: application/json\r\n\
+            user-agent: $USERAGENT\r\n\
             accept-encoding: gzip\r\n\
             host: $HOST\r\n\
             \r\n\
@@ -142,9 +142,9 @@ fn test_accept_encoding_header_is_not_changed_if_set() {
     let server = server! {
         request: b"\
             GET /accept-encoding HTTP/1.1\r\n\
+            accept-encoding: identity\r\n\
             user-agent: $USERAGENT\r\n\
             accept: */*\r\n\
-            accept-encoding: identity\r\n\
             host: $HOST\r\n\
             \r\n\
             ",