Leading/trailing spaces between runs are not preserved

[AlexanderVatov]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:
p.add_run("A ");
p.add_run("run");
p.add_run(" test");

Expected behavior
"A run test" is expected; "Aruntest" is obtained. The leading and trailing spaces are there in the XML, but they are not preserved unless xml:space="preserve" is added to the "w:t" tag.

I plan to fix this myself; I'm opening an issue as requested in Contributing.md.