Fix Regression where variables passed in graphql HOC options were not merged with mutation variables

[samginn]

Prior to 2.0, variables passed in via the graphql HOC were merged with the variables passed into the mutation function directly. After 2.0, this no longer happens and variables included within the graphql HOC are dropped. This resulted in a breaking change for my app as we use this pattern consistently across the codebase.

Here's an example:

const Component = ({ mutate }) => {
   return (<div onClick={() => mutate({ variables: { second: 2 } })}>Hi</div>)
}

graphql(MUTATION_DOCUMENT, {
  options: () => ({ variables: { first: 1 } })
})(Component)

In this example, the mutation would only receive the variable second: 2 and fail to incorporate the variable first: 1. If you do not include any variables within the mutate call (just call mutate()), then first: 1 is successfully included, but when including any variables within mutate, they are overwritten rather than merged. This PR reverts that behavior to where they will be merged together.

I've included a test to test for this scenario as well.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• If this was a change that affects the external API used in GitHunt-React, update GitHunt-React and post a link to the PR in the discussion.

[apollo-cla]

@samginn: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[hwillson]

Thanks for working on this @samginn!