BUG: Series dot product __rmatmul__ doesn't allow matrix vector multiplication
- [x] closes #21530 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

the new `__rmatmul__` implementation in Series seems to be missing matrix vector multiplication case as raised in the issue in question. Only inner product of two vectors was supported in `__rmatmul__` method.

This PR uses the same implementation in DataFrame to add support in Series and test case.

matmul operator (`@`, `@=`) was added in python 3.5 in https://www.python.org/dev/peps/pep-0465/

```python
class A(object):
    def __matmul__(self, other):
        print('__matmul__ is called in A.')

class B(object):
    def __rmatmul__(self, other):
        print('__rmatmul__ is called in B.')

A() @ B()
B() @ A()
del A.__matmul__
A() @ B()

>>> A() @ B()
__matmul__ is called in A.
>>> B() @ A()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for @: 'B' and 'A'
>>> del A.__matmul__
>>> A() @ B()
__rmatmul__ is called in B.
```