curl: handle multiple Content-Type headers

`#curl_http_content_headers_and_checksum` contains code that works
with a `Content-Type` header in a response but it expects there to
always be only one header in the response. This is normally a
reasonable assumption but we've come across a server that is giving
a response with multiple `Content-Type` headers in the response, so
this produces an error and causes `brew audit` to fail when checking
the URL.

This works around the issue by naively using the last `Content-Type`
header in the response when there's more than one. It's not something
that should normally occur but this will handle the situation when it
does.

Library/Homebrew/utils/curl.rb

@@ -467,9 +467,15 @@ module Utils
 
       if status.success?
         open_args = {}
+        content_type = headers["content-type"]
+
+        # Use the last `Content-Type` header if there is more than one instance
+        # in the response
+        content_type = content_type.last if content_type.is_a?(Array)
+
         # Try to get encoding from Content-Type header
         # TODO: add guessing encoding by <meta http-equiv="Content-Type" ...> tag
-        if (content_type = headers["content-type"]) &&
+        if content_type &&
            (match = content_type.match(/;\s*charset\s*=\s*([^\s]+)/)) &&
            (charset = match[1])
           begin