[contents] Fix parsing of incomplete headers

Response headers may contain fields with no values.

Example:
  "Referrer-Policy: "

In this case the current implementation of explode() results in an
error because there is no content after ": ". Changing the delimiter
to ":" and trimming the value manually fixes that issue.

lib/contents.php

@@ -322,8 +322,8 @@ function parseResponseHeader($header) {
 				$header['http_code'] = $line;
 			} else {
 
-				list ($key, $value) = explode(': ', $line);
-				$header[$key] = $value;
+				list ($key, $value) = explode(':', $line);
+				$header[$key] = trim($value);
 
 			}