Failing to parse non-standard line breaks

Hi,

A while ago I have found that the package does not handle non standard line breaks e.g. `\n` for separation of multiple files. Both, Flask and `https://httpbin.org/post` can handle non-standard line breaks and it would be nice if `python-multipart` would do so too.

The issue was originally reported here: https://github.com/encode/starlette/issues/1059

However, I think this is now a `python-multipart` and not a starlette issue, so I am forwarding the issue here.

No source file or test is named. Start by reproducing a multipart request whose parts are separated with LF line breaks, then inspect the parser's handling of line delimiters and compare it with Flask and httpbin.org/post. Done means python-multipart successfully parses the non-standard line breaks without regressing standard multipart input.

Written by the indexing model from the issue text.