Fixed njs_value_index().

Fixed njs_value_index().

Previous, there were several issues with njs_values_hash_test().
1) The function assumed string types of left and right values are identical.
   (If current value is a long string it assumed the second value is also
   a long string). Long vs short strings collision.
2) The function assumed if hashes are identical value length are equal.
   Long vs long string collision.

The fix is to always check value sizes.

In addition, for short strings njs_value_index() calculated hash value including
padding bytes (which values are undefined). It could result in identical
short strings (but with different padding bytes) treated as different strings.