Use the :encoding arg on Document.parse even when contents are blank

[batter]

Currently, Nokogiri::XML::Document.parse and Nokogiri::HTML::Document.parse discards the encoding argument if it receives nil or blank contents for the string_or_io argument. This pull tweaks the code to set the encoding value (if it is passed in) even when the string_or_io argument is blank.

# before this pull request
doc = Nokogiri.HTML(nil, nil, 'UTF-8')
doc.encoding # => nil
# after this pull request
doc = Nokogiri.HTML(nil, nil, 'UTF-8')
doc.encoding # => 'UTF-8'

This is really just a convenience thing, so that if you want to compose a blank document with an particular encoding, you can do something like this:

doc = Nokogiri.HTML(nil, nil, 'UTF-8')
Nokogiri::HTML::DocumentFragment.new(doc, string)

instead of having to do something like this:

doc = Nokogiri::HTML::Document.new
doc.encoding = 'UTF-8'
Nokogiri::HTML::DocumentFragment.new(doc, string)

This is particularly helpful for Ruby18 users where the String class does not have the concept of encoding on it's own. True, you can technically condense the above example down into this:

doc = Nokogiri::HTML::Document.new.tap { |d| d.encoding = 'UTF-8' }
Nokogiri::HTML::DocumentFragment.new(doc, string)

But it's cleaner to just have the parse method enforce the encoding regardless of contents it receives.

[knu]

Looks good to me, but we are already in RC period.
What do you think, @flavorjones?

[flavorjones]

I'm very sorry to not have responded to this long ago.

This seems like a reasonable change. Will make one comment inline.

[flavorjones]

I imagine, given the time frame of the submission, that this was intended to exclude 1.8 and not to exclude 2.0+. Correct?

[flavorjones]

Merged manually. Thank you for this contribution!