fix tests for v0.4

With v0.4 Char is no longer a subtype of Integer
Support v0.4 style association construction via Pair operator

Also add warning that old style construction is deprecated, same as Dict()
use Base.warn_once() instead of warn()
filter on VERSION >= '0.4.0-dev+980'
Merge pull request #66 from srp/pair-syntax

Support for new v0.4 pair syntax