Hashie::Mash existence methods should always delegate to @result._source

[emptyflask]

Before:

@result.title #=> "Example"
@result.title? #=> false
@result._source.title? #=> true

With this change:

@result.title #=> "Example"
@result.title? #=> true
@result._source.title? #=> true

[emptyflask]

We might have to rethink this, if highlight? and facets? are expected to work as well.

[karmi]

Facets are on the main response object, but highlight is a valid question...

[karmi]

@emptyflask Merged the commits, thanks!

[karmi]

Grr Github formatting pedantry, Closes: #64 should have been Closes #64...