Avoid trimming elements with the text content

[r-glazkov]

Trailing whitespaces in FB2 style elements should not be trimmed. It's fine to trim when parsing other non-span elements through. Looks like the proper fix should land in tafia/quick-xml#285.

While the quick-xml trims strings, we may try to fix the output, in a best-effort manner, by exploiting some heuristics:

• consecutive whitespaces are merged in a single one
• an element pair where the right element has a baseline shift (subscript, superscript) should be joined without a whitespace