Do not allow non-hash values in HashOf contract

[abevoelker]

Given these HashOf contracts:

Contract HashOf[Symbol => String] => Any
def foo(args)
end

Contract KeywordArgs[:hash => HashOf[Symbol => String]] => Any
def bar(args)
end

calling foo(nil) or bar(hash: nil) results in NoMethodError: undefined method``keys' for nil:NilClass

[abevoelker]

Actually I amended the commit since it wasn't really a thorough fix - you could pass in pretty much anything that isn't a Hash to trigger the error, such as foo(1). I don't know if is_a?(Hash) is too strict of a check or whatnot so let me know if there is a better way to do it.

[waterlink]

LGTM

[waterlink]

@abevoelker Good catch! Thanks!