Merge between Preformatted and Paragraph blocks is removing all newlines.

[SergioEstevao]

Describe the bug
When merging a paragraph block to a preformatted block all the newlines are removed

To reproduce
Steps to reproduce the behavior:

1. Start a new post
2. Add a preformatted block and write some content like this:

One
Two
Three

3. Add a Paragraph block and write for example: Hello
4. Put the cursor before Hello, and press backspace
5. See the the content in the preformatted block all merged witthout new lines and multiple spaces:
   One Two Three Hello

Expected behavior

The block should be merge maintaining all the new lines and spaces.

Screenshots

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser Safari
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser Safari
• Version [e.g. 22]

Additional context
Gutenberg version: 6.9.0 rc1