protect headers from newline

[rinfex-holfer]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
New tests added?	not yet
Fixed tickets	#1060
License	MIT

Description

This is temporary fix for #1060 issue, I did it for a project in which I participate. It must be imporoved of course, because this bug involves not only <h*> elements, but every block-type elements.
It will be good if someone will give ideas for improvements and tests for this PR, I'm afraid to break something).

[nmielnik]

@Greeny7 thanks for opening this and apologies for the delay in getting some feedback.

1. Take a look at this part of content.spec.js. There you can find some example tests that test some of the code around where your change is. I would definitely recommend adding some tests to further verify the functionality you're adding. To run the tests, just run grunt from the command line.
2. Since you're calling getClosestBlockContainer() to get the first block element, you might be able to rip out the existing code that checks if the node var is a block element or part of an <li>. You may just be able to get the element returned from calling getClosestBlockContainer() and see if you get a result other than the editor element itself. If it's not the editor <div> then you know there was no block container and it's ok to allow the <p> to be inserted. However, there might be some elements that we want to be able to have a nested <p> element inside of it (<blockquote> elements come to mind for some reason) so we might want to think about allowing other block containers to still contain a <p>, but seeing how everything work if you just inspect the block container would definitely be worth it if you're can try it out.