http: Added proxy tunnel authentication message header value extraction

...following recent changes to Curl_base64_decode() rather than trying
to parse a header line for the authentication mechanisms which is CRLF
terminated and inline zero terminate it.

lib/http.c

@@ -187,7 +187,7 @@ char *Curl_checkheaders(struct SessionHandle *data, const char *thisheader)
  * case of allocation failure. Returns an empty string if the header value
  * consists entirely of whitespace.
  */
-static char *copy_header_value(const char *h)
+char *copy_header_value(const char *h)
 {
   const char *start;
   const char *end;