Adds GetAll method

GetAll returns all stored values for the request as a map.
Adds test for GetAll for existing request
Adds test for GetAll for empty request
Return empty map when there are no values
Adds GetAllOk
Remove GetAllOk method and GetAll now returns nil for invalid request
Return map instead of pointer value

As kisielk just [told me](https://github.com/pjvds/context/commit/709e005f84167f4206737b8ba6c560d1ebb18b85#commitcomment-3636470), maps can be nil
Fix docstring
Add tests for GetAllOk
Merge pull request #11 from adammck/add_getall

Add GetAll method (take two)