Fix new Context handling (react@^16.6.x)

[SleepWalker]

As of react 16.6 the both Provider and Consumer types has _context property, which leads to context loss whenever we are visiting Consumer:

if (currentElement.type._context) {
  // eslint-disable-next-line no-param-reassign
  currentElement.type._context._currentValue =
    currentElement.props.value
}

The most naive way to fix it is to check whether we actually have value prop. There is more advanced approach here: jaydenseric/graphql-react@v3.0.0...v4.0.1#diff-3e6d5558503509cddfcf7ef1bc7ee575

Here is a demo of an error: https://codesandbox.io/s/o4wrj5m0jy