[Musicxml export] Preserve whitespace in TextBox

[jacobtylerwalls]

I found that xml:space="preserve" is the way to preserve the display of newlines in a <credit-words>.
music21 was splitting text content on newlines and creating distinct <credit-word> elements, but without changing the position attributes, they were superimposed:

Before

After

[coveralls]

Coverage decreased (-0.0002%) to 93.054% when pulling 8b29eee on jacobtylerwalls:credit-words-fix into c46bde4 on cuthbertLab:master.

[mscuthbert]

Oh, this is so much better. (Btw -- what piece is that? I have a vague memory of encoding it, but no clue!)

[jacobtylerwalls]

It's the Amy Beach choral work in the corpus.