[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

@@ -323,7 +323,7 @@ function parseResponseHeader($header) {
 			} else {
 
 				list ($key, $value) = explode(':', $line);
-				$header[$key] = $value;
+				$header[$key] = trim($value);
 
 			}