`string[]=(idx, replace)` should return `replace`.
## Actual (wrong)
```
string.[]=(idx, replace) → string
string.[]=(idx, len, replace) → string
```
## Expected
```
string.[]=(idx, replace) → replace
string.[]=(idx, len, replace) → replace
```
## Sidenote
As of the current mruby-compiler, `(string[idx] = 'X')` creates not only "CALL_NODE" but also "ASGN_NODE" and "OP_MOVE", overriding the wrong return value.
On the other hand, `string.[]=(idx, 'X')` creates only "CALL_NODE", exposing the wrong return value.
If my new mruby-compiler2, leveraging Prism, took the place of official compiler, `(string[idx] = 'X')` and `string.[]=(idx, 'X')` would be going to generate the same VM code without "OP_MOVE".
So I paranoidly added tests.
FYI: You can find how the new mruby-compiler2's AST and VM code look like in mruby/c's issue (mruby/c had the same bug): mrubyc/mrubyc#210