Enable support for "return-empty"

[geoffoxholm]

This small change adds support to get empty cells in the cell range.

[magnetikonline]

Hello @lafcadio - thanks for the PR.

I'll review this work and get it merged - when time allows!

Pete

[magnetikonline]

@lafcadio I'm curious to know - where did you find return-empty documented at the official Google API docs page? :)

[geoffoxholm]

@magnetikonline - It's hinted at all over the place, but I agree, it's kind of illusive. Here's a link:
https://developers.google.com/gdata/javadoc/com/google/gdata/client/spreadsheet/CellQuery

[magnetikonline]

Thanks @lafcadio - champion, I'll add that to the README.md when I finally get around to merging (sorry, been a busy time in life!).

Pete

[geoffoxholm]

No worries, @magnetikonline, thanks for your consideration.

[adriweb]

Note @lafcadio: There needs to be $value = $cellRangeCriteriaList[$key]; right before line 171 of the proposed patch.

[magnetikonline]

Thank @adriweb - I'll take that into consideration.

[geoffoxholm]

Yeah, @adriweb is right. Whoops. Not sure how that happened. Thanks.

[magnetikonline]

Hey @geoffoxholm - finally getting around to some of these.... have committed your fine work as part of 58773de.