Cleanup after #2002

[bmarty]

Following what have been done on #2002, tests had to be updated.

[bmarty]

@MexHigh I'm not sure that removing \n from HTML code in #2002 was source of bugs. Could you confirm please?

[tulir]

Any bugs that appear to be caused by removing \n from outgoing HTML are actually caused by whatever breaks when receiving HTML without \n. Other clients might remove \n even if you don't, so the bug needs to be fixed in the handling, not the sending.

Also as I noted in #2002 (comment), at least two of the four mentioned issues were not fixed.

[bmarty]

The more I think about it, the more I have the feeling that having a body and a formatted_body is really a bad solution. The text itself is duplicated (and can even be totally different...), and it's not possible to handle all the possible cases without bug: save draft, edit message, etc.

[bmarty]

Anyway @tulir I think you are right, I will revert some change and reopen closed issues by #2002, until we can confirm that they are fixed.

[MexHigh]

One of the main problems with removing all \n from a message was that if it contains not only code, but normal text as well, the linebreaks in the plaintext would be removed too. It actually causes two inline code blocks with a linebreak between them to appear side by side. Another solution would be to replace \n with <br> tags but they are not used in "normal" messages either.
Linebreaks don't get removed on Element Web too, so I've oriented myself towards that.
As @tulir mentioned, the two issues (#350 and #1375) were not fixed, because I misunderstood what the actual problems were. My bad.

[bmarty]

@MexHigh thanks.
I will revert the change on line break then (in this PR). And add unit test to cover the case you are talking about.