accounts/abi: fix set function

[zhiqiangxu]

Without this patch, this test will report abi: cannot unmarshal string in to string:

After this patch, the above test will pass.

After all, there's no point to call set on dst.Elem() if dst.Elem() can not be set (CanSet returns false)

[zhiqiangxu]

Could you please:

• [x] Fix the geth test that this PR breaks
• [x] Add a simpler test into this PR that reproduces the issue. The test you provided belongs to a different repository, and does way more things than needed to reproduce the issue.

Done. Basically now you can unpack a solidity string into both []interface{}{""} and []interface{}{new(string)}, previously you can only unpack it into []interface{}{new(string)}(the indirection is mandated,or you'll get an error like "abi: cannot unmarshal string in to string", which is technically unnecessary).

The same for other types like uint8 etc.

[holiman]

@gballet lgty ?