Added "called" prop to the Mutation component

[excitement-engineer]

Added a "called" field in the MutationResult prop returned in the render prop of the Mutation component. This field is true or false depending on whether the mutation has been called previously.

Adding this field ensures that the MutationResult object in the render prop is always defined which makes destructuring easier. In addition, this will prevent confusion about the meaning of an undefined MutationResult in the render prop as mentioned by #1773.

Would love to hear some feedback regarding these changes. Does this improve the experience of using the Mutation component.

[fbartho]

This is a breaking API change, but it really makes sense to me. I support this!

[janhartmann]

This is so much needed - I was upgrading our system to the beta version, but I ran into these undefined issues but could not understand why, after viewing the source code of the Mutation component.

It was because this change is not yet released. I can not wait! Nice work, guys!