Fix for ruby 2.0

In Ruby 2.0, respond_to? no longer returns true for protected methods unless you explicitly pass true as a second argument. This causes try_send to fail for @content blocks, keeping sass from knowing that a mixin supports blocks.

Also, Kernel.warn behaves slightly differently. Since the difference is in number of newlines, I figure it's a cosmetic change and made the most expedient fix I could think of.

Let me know if you see any issues!

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This fix is entirely from [sax](https://github.com/sax) and closes #572 but since that issue has not been updated for a month I thought I'd fix the things commented on in that thread.