Clean up javadocs.
Match MRI error text.
Add to_a and to_ary cases.
Clean up imports.
Fix new coercion logic.

MRI's tests for respond_to? also included a test for NOEX_BASIC,
which appears to be a flag only added to respond_to?,
respond_to_missing?, and method_missing. We do not have this flag,
so instead we just check that respond_to? is not the builtin
version.