URL: warn if key doesn't exist in url drop

[ghost]

fixes #5332

the tests only cover not throwing a NoMethodError because i can't figure out how to capture the logged message to check if it logged the correct message (minitest's capture_io only works with kernel puts, weirdly enough)

[DirtyF]

/cc @jekyll/core

[parkr]

Instead of a begin .. rescue, what about checking for the key first, like

template.gsub(%r!:([a-z_]+)!) do |match|
  key = match.sub(":".freeze, "".freeze)
  raise NoMethodError, "No URL template key #{key} exists!" unless @placeholders.key?(key)
  if @placeholders[key].nil?
    "".freeze
  else
    self.class.escape_path(@placeholders[key])
  end
end.gsub(%r!//!, "/".freeze)

[ghost]

hmm, that makes more sense, yeah

[ghost]

hmm, by using this approach, it looks like the URLDrop (at least the one i use in my tests) throws an unrelated error because it can't find fallback_data when using key? or #[]

[parkr]

Ah, ok. No problem. Define def fallback_data in UrlDrop as an empty hash.

class UrlDrop
  private
  def fallback_data
    {}
  end
end

[ghost]

updated according to @parkr's comments

[parkr]

LGTM!

[ghost]

@jekyll/core is this ok to merge? its been sitting for a while

[parkr]

@jekyllbot: merge +minor