fix: assign properties of mutations in withMutation

[michael-small]

Description

Would fix #286

Would fix the following:

// component
readonly myMutation = this.store.myMutation; // Mutation<number, any>

// html
{{ myMutation.isPending() }}

// runtime error:
ERROR TypeError: ctx.myMutation.isPending is not a function

Object.assign will explicitly copy the properties of the mutation so that they are all available

Outstanding before going out of Draft

Tests pending