Fix parameter coerce when Hash or Array expected.

[serargz]

When Array is expected and for some reason a Hash is received, an
unhandled exception was raised. The same thing happens when Hash is
expected an Array is received. This commit converts the Hash to Array
with Array#to_a for the first case, and converts the Array to string for
the former.

[serargz]

hi @mattt, any plans to merge this PR?

[mattt]

Apologies for the delay in responding, @serargz. Thank you for your contribution.

I think there's a lot of room for improvement in the current implementation. The clever Ruby idioms I used in the initial implementation aren't looking too great now with all of this additional functionality. I'm investigating a refactoring for all this in the next major release, and will look into how best to incorporate your changes.