Fix bug causing a wrong 'Host' header

When reusing a curl handle on which the 'Host' header was explicitly
set, the previously-set header was being kept in use for following
requests.

The issue was in curl-impersonate's merging of user-supplied headers
with its own list of browser headers. The call to
Curl_http_merge_headers() which takes care of this had been placed after
the handling of the host header, which caused the previous one to be
used.

firefox/patches/curl-impersonate.patch

@@ -488,7 +488,7 @@ index 04871ad1e..cd5998146 100644
    {"HTTPPOST", CURLOPT_HTTPPOST, CURLOT_OBJECT, 0},
    {"HTTPPROXYTUNNEL", CURLOPT_HTTPPROXYTUNNEL, CURLOT_LONG, 0},
 diff --git a/lib/http.c b/lib/http.c
-index f08a343e3..879151dd2 100644
+index f08a343e3..2bbce4b23 100644
 --- a/lib/http.c
 +++ b/lib/http.c
 @@ -84,6 +84,7 @@
@@ -636,16 +636,16 @@ index f08a343e3..879151dd2 100644
  CURLcode Curl_http_useragent(struct Curl_easy *data)
  {
    /* The User-Agent string might have been allocated in url.c already, because
-@@ -3067,6 +3163,11 @@ CURLcode Curl_http(struct Curl_easy *data, bool *done)
-   if(result)
-     return result;
+@@ -3063,6 +3159,11 @@ CURLcode Curl_http(struct Curl_easy *data, bool *done)
+   http = data->req.p.http;
+   DEBUGASSERT(http);
  
 +  /* curl-impersonate: Add HTTP headers to impersonate real browsers. */
 +  result = Curl_http_merge_headers(data);
 +  if (result)
 +    return result;
 +
-   result = Curl_http_useragent(data);
+   result = Curl_http_host(data, conn);
    if(result)
      return result;
 diff --git a/lib/http2.c b/lib/http2.c