The index variable in Signature.validate() was incremented at the
end of the for loop, but the capture group was read using the
pre-increment value. When the first argument is an array (type 'a')
and the second is a function (type 'f'), the function argument was
validated against group(1) instead of group(2), causing a false
T0410 signature mismatch error.

Fix: use post-increment (index++) inline at the point where the
capture group is read, and remove the redundant index++ at the
end of the loop.

Also fix DateTimeTest.testToMillis assertion to be timezone-agnostic.

Fixes dashjoin#79