Optimize path sanitization of default front matter

[ashmaroli]

Summary

String#gsub duplicates the given string irrespective of a pattern match.
So, it is better if one checks if a given string will match the pattern beforehand and return the original string if it doesn't match the pattern.

Regarding the pattern, %r!\A/|(?<=[^/])\z!, it translates to
Test if a given string either starts with / or just match the end-of-string preceded by a character that is not /.
The second option of the regex will always generate a match for the major use-cases
(e.g. scope["path"] == "pages" or scope["path"] == "\pages")

So we can optimize for the major use-cases by checking if the given string start_with? "/" prior to the gsub call.

[ashmaroli]

@jekyllbot: merge +dev