Fix deprecation message for missing doc method

[ashmaroli]

• This is a 🐛 bug fix.
• I've added tests.

Summary

The deprecation message issued when a key in document's data is called directly currently points to the Jekyll gem.
It should instead point to the offending location in the concerned plugin.

[parkr]

@ashmaroli the cucumber test matches single lines. What does the full output look like?

[parkr]

What does the full output look like?

Running a cucumber scenario with DEBUG=true in the command line outputs the Jekylll build output:

Configuration file: none
       Deprecation: Document#title is now a key in the #data hash.
                    Called by /Users/parker/code/jekyll/jekyll/tmp/jekyll/_plugins/foo.rb:2:in `block in <top (required)>'.
       Deprecation: Document#title is now a key in the #data hash.
                    Called by /Users/parker/code/jekyll/jekyll/tmp/jekyll/_plugins/bar.rb:3:in `dummy?'.

LGTM!

[parkr]

@jekyllbot: merge +fix