Add heredoc test to StringReplaceRector

[koertho]

This PR add a heredoc test to StringReplaceRector to catch the case in #31
I hope I got it right.

[zoglo]

It's not exactly what I meant by #31 (comment).

The test itself is just something to validate the Rector Rule and has no effect on the outcome of the refactoring.

[zoglo]

What I meant is that we can explicitly skip the DOC types or we could include them by not rewriting the node but actually changing the value and thus triggering a repaint of the $node itself (keep in mind that changing attributes would need a manual repaint but that's not needed in this case).

Refactoring the string with the indent can be tricky and needs to be checked if it's possible. I do know that there is a DOC_INDENTATION attribute but that one is bound to the identifier, not the string itself.

TL;DR:
Basically using $node->value and returning the original node instead of returning a new string within the StringReplaceRector.

[koertho]

then we misunderstood each other :D
I will have a look into it next week.

[aschempp]

ping @koertho