Fix non-idempotent setext heading whose line ends in a tab

[sarathfrancis90]

I noticed mdformat isn't idempotent for a setext heading whose line ends in a tab. Formatting it once gives a heading with two spaces, and formatting that output again collapses them to one:

import mdformat
mdformat.text("first\tsecond\n---")      # -> "## first  second\n"  (two spaces)
mdformat.text("## first  second\n")        # -> "## first second\n"   (one space)

A line of a setext heading can end in a tab, which the parser keeps in the inline text token. When the heading is rewritten as a single-line ATX heading, that tab turns into a space right next to the space produced for the line break, so two spaces sneak in. A trailing space (rather than a tab) doesn't hit this because the parser strips trailing spaces.

The fix collapses the spaces around the converted line break so the heading renders the same way on a second pass. Added a fixture case to default_style.md; full test suite, flake8, black, isort and mypy are green.