Relative posts should never fail to build, even if @dir or @name is nil

Fixes https://github.com/jekyll/jekyll/issues/1963
DRY up code, props @tamouse
First draft
Fix group_by_filter test by adding 2 items
Fix partial test
Upgrade gemspec
Make sort test run
Give File.join the strings from the array
Add comment about symlinked directories
Default to using the UTF-8 encoding when reading files.

Fixes #2029.
Added a breakpoint for .site to make it responsive
Removed accidental linebreak
Merge pull request #2038 from joshbrw/master
Update history to reflect merge of #2038 [ci skip]
Merge pull request #2031 from jekyll/utf8-heart
Update history to reflect merge of #2031 [ci skip]
Update history to reflect merge of #1976
Merge pull request #1976 from jekyll/fix-pagination-issue

Relative posts should never fail to build, even if @dir or @name is nil
Merge pull request #2009 from penibelst/test-sass-include
Update history to reflect merge of #2009 [ci skip]