Incorrect space-padding in code span rule

[martincizek]

As CommonMark spec 6.3 states, a single space character is removed from the front and back if the string both begins and ends with a space character (shortened).

But rules.code adds spaces independently, which means false spaces can be introduced into the resulting code span.

The corresponding test is actually expecting a wrong value adding a leading space to code when converted back:

  <div class="input"><code>`starting with a backtick</code></div>
  <!-- currently expected -->
  <pre class="expected">`` `starting with a backtick``</pre>

While the right output is:

  <!-- should be expected -->
  <pre class="expected">`` `starting with a backtick ``</pre>

---

Also, the current rule is written quite robust in several aspects - trim() suggests that whitespace-only content may come in and multiline regexp flag suggests that even newlines may come in. None of that is really relevant in current Turndown, but to keep the rule robust:

• There is no reason to reject non-empty whitespace-only string - it would make also a valid code span if it wasn't rejected by isBlank and wasn't trimmed to empty string before.
• Leading and trailing space should be also considered for decision regarding extra space if the incoming content was not trimmed before.
• Newlines should be converted to spaces if they were not converted by collapse-whitespace.js before.

Disclosure: our next contributions are going to build on this robustness.