Add raising send() method to specs

This ensures that these methods aren't called when sending read /
readpartial. This is because for example UNIXSocket has a send() method
and this verifies the behavior.
Make sure to use a method send and not call a send() method

UNIXSocket for example has a send() method that shouldn't be called.
Add spec for not seeking when no offset is given

Extracted from #2755
Don't call IO#pos if no offset is given

Fixes #2755