Trailing/leading whitespace in emphasis/strong spans isn't serialized correctly

[marijnh]

I.e. you'll end up with Markdown like foo * bar* which the Markdown parser won't actually recognize as emphasis because there's whitespace after the asterisk.

Originally reported here

[TimothyGu]

In this situation, rather than outputting invalid Markdown, it might be better to move the whitespace in the emph/strong span out of the span (i.e. foo  bar)

[owjsub]

This issue commonly affects iOS/Android users. They are more likely to add a space before toggling emphasis.