MDL-33955 fix sloppy whitespace in curl class
MDL-33955 make downalod_file_content() use curl class and cleanup

The options in constructor were renamed to setttings in order to prevent confusion.
This removes ignored CURLOPT_BINARYTRANSFER, add newline header cleaning,
reworks proxy settings and some other minor issues.
MDL-33955 add file download and curl unit tests