I don't see anything automatic; it's a pain. I do:
- Click on the upper-right menu in the message you received.
- Select 'show original'.
- You get the raw text, including headers, etc. Scroll down.
Thiis is not so great for emails that have lots of images, etc. But you just want the text. The alternative is to cut and paste into a text editor.
I looked at a raw message. The text would indeed be on a single line: it's coded with '='s joining the lines. It's a single object. Gmail does the correct thing. It's the sender's client that creates the "problem". It would be nice if gmail had a right-click option so you could explicitly split a string...