Crash if you call String#start_with/end_with with non-String arguments.

On OpenBSD/amd64 5.3,
```
% bin/mruby -e 'p "str".end_with?(:a)'
zsh: segmentation fault (core dumped)  bin/mruby -e 'p "str".end_with?(:a)'
% bin/mruby -e 'p "str".end_with?(1)'
zsh: segmentation fault (core dumped)  bin/mruby -e 'p "str".end_with?(1)'
```

Note: patch may conflict with #1484 and #1485.