Fix space-padding in code span rule

[martincizek]

Code span rule now exactly conforms to the spec.

Quality assurance:

• Fixed expectation in a test expecting a wrong output.
• The fixed test matches fixed output.
• Code should now be more robust in terms of future development.

Performance:

• Saved one trim()
• Executing one more regexp (roughly).
• Less code. :)

From the commit message:

• Follow CommonMark spec more precisely
• Make the code span rule even more robust for future Turndown development.
• Fix Incorrect space-padding in code span rule #316.

[martincizek]

@pavelhoral Asking for a code review, thanks.

[pavelhoral]

LGTM