Give Dancer2::Test the ability to handle multiselect inputs

Dancer2::Test treats key => value params as always being scalar values.
However, it is possible to send multiple values per key in an http form.
multiselects/checkboxes etc. This gives Dancer2::Test the ability to
handle that (Dancer2 supports this fine, it's just the tests that don't)
Merge pull request #220 from shumphrey/dancer-test-multiselect

Give Dancer2::Test the ability to handle multiselect inputs