BUG: Make sum & mean method's output dtypes behave like corresponding numpy methods.

This is to address [issue 2534 ](https://github.com/scipy/scipy/issues/2534) so these methods will act like the corresponding numpy methods. 

Note that this will change behavior, and could break code that relies on self.dtype being the default dtype. 

Also, there are no tests in this PR, I was writing new tests when I found this bug. And this PR is required for them to pass. But these tests are associated with adding tests for the new bool dtype, and other existing dtypes which did not have tests.