Change in the Differ broke block quote tests

[Reinmar]

https://travis-ci.org/ckeditor/ckeditor5/builds/345037426

Fails since ckeditor/ckeditor5-engine@e70ab96

With:

Chrome 64.0.3282 (Linux 0.0.0) BlockQuoteCommand execute() applying quote should wrap a single block FAILED
	AssertionError: expected '<p>abc</p><blockquote><p>x{}x</p><p>xx</p></blockquote><p>def</p>' to equal '<p>abc</p><blockquote><p>x{}x</p></blockquote><p>def</p>'
	    at Context.it (build/.automated-tests/entry-point.js:86501:144)
Chrome 64.0.3282 (Linux 0.0.0) BlockQuoteCommand execute() applying quote should wrap multiple blocks FAILED
	AssertionError: expected '<blockquote><h>a{bc</h><p>xx</p><p>de}f</p><h>abc</h><p>xx</p><p>def</p></blockquote>' to equal '<blockquote><h>a{bc</h><p>xx</p><p>de}f</p></blockquote>'
	    at Context.it (build/.automated-tests/entry-point.js:86524:144)
Chrome 64.0.3282 (Linux 0.0.0) BlockQuoteCommand execute() applying quote should merge with an existing quote FAILED
	AssertionError: expected '<blockquote><h>a{bc</h><h>abc</h><p>x}x</p><p>yy</p></blockquote><p>def</p>' to equal '<blockquote><h>a{bc</h><p>x}x</p><p>yy</p></blockquote><p>def</p>'
	    at Context.it (build/.automated-tests/entry-point.js:86548:144)
Chrome 64.0.3282 (Linux 0.0.0) BlockQuoteCommand execute() applying quote should not merge with a quote preceding the current block FAILED
	AssertionError: expected '<blockquote><p>abc</p></blockquote><blockquote><p>x{}x</p><p>xx</p></blockquote>' to equal '<blockquote><p>abc</p></blockquote><blockquote><p>x{}x</p></blockquote>'
	    at Context.it (build/.automated-tests/entry-point.js:86567:144)
Chrome 64.0.3282 (Linux 0.0.0) BlockQuoteCommand execute() applying quote should not merge with a quote following the current block FAILED
	AssertionError: expected '<blockquote><p>x{}x</p><p>xx</p></blockquote><blockquote><p>abc</p></blockquote>' to equal '<blockquote><p>x{}x</p></blockquote><blockquote><p>abc</p></blockquote>'
	    at Context.it (build/.automated-tests/entry-point.js:86587:144)
Chrome 64.0.3282 (Linux 0.0.0) BlockQuoteCommand execute() applying quote should merge with an existing quote (more blocks) FAILED
	AssertionError: expected '<blockquote><h>a{bc</h><p>def</p><h>abc</h><p>def</p><p>x}x</p></blockquote><p>ghi</p>' to equal '<blockquote><h>a{bc</h><p>def</p><p>x}x</p></blockquote><p>ghi</p>'
	    at Context.it (build/.automated-tests/entry-point.js:86613:144)
Chrome 64.0.3282 (Linux 0.0.0) BlockQuoteCommand execute() applying quote should not wrap non-block content FAILED
	AssertionError: expected '<blockquote><p>a{bc</p><p>abc</p></blockquote><widget>xx</widget><blockquote><p>de}f</p><p>def</p></blockquote>' to equal '<blockquote><p>a{bc</p></blockquote><widget>xx</widget><blockquote><p>de}f</p></blockquote>'
	    at Context.it (build/.automated-tests/entry-point.js:86638:144)
Chrome 64.0.3282 (Linux 0.0.0) BlockQuoteCommand execute() applying quote should correctly wrap and merge groups of blocks FAILED
	AssertionError: expected '<blockquote><p>a{bc</p><p>abc</p></blockquote><widget>xx</widget><blockquote><p>def</p><p>def</p><p>ghi</p></blockquote><widget>yy</widget><blockquote><p>jk}l</p><p>jkl</p></blockquote>' to equal '<blockquote><p>a{bc</p></blockquote><widget>xx</widget><blockquote><p>def</p><p>ghi</p></blockquote><widget>yy</widget><blockquote><p>jk}l</p></blockquote>'
	    at Context.it (build/.automated-tests/entry-point.js:86664:144)
Chrome 64.0.3282 (Linux 0.0.0) BlockQuoteCommand execute() applying quote should correctly merge a couple of subsequent quotes FAILED
	AssertionError: expected '<p>x</p><blockquote><p>a{bc</p><p>abc</p><p>def</p><p>ghi</p><p>jkl</p><p>mn}o</p></blockquote><p>y</p>' to equal '<p>x</p><blockquote><p>a{bc</p><p>def</p><p>ghi</p><p>jkl</p><p>mn}o</p></blockquote><p>y</p>'
	    at Context.it (build/.automated-tests/entry-point.js:86699:144)