[Issue 1649] Variant coercion fails with delegates

[Brad Roberts]

http://www.dsource.org/projects/phobos/browser/candidate/phobos/std/variant.d?rev=511

Why static assert(false) rather than return false?  Additionally, if 
you're going to static assert(false), then a message ought to be included 
since it's going to result in a nice meaningless compilation failure.

Additionally.. add a code block -> add a unit test!

Lastly, stop doing trivial bug fixes on the candidate branch.  They belong 
directly on the trunk.