Add Flow type definitions for node-mocks-http to improve Flow coverage

[alxmyth]

Type of issue

Missing Flow type definitions

Description

src/mock-context.js has low Flow type coverage due to missing type definitions for node-mocks-http.

Current behavior

Low Flow coverage

Expected behavior

100% Flow coverage

Steps to reproduce

1. npm install -g flow-coverage-report
2. flow-coverage-report -i 'src/**/*.js' -i 'src/**/*.jsx' -x 'src/test/**' -t html -t json -t text --threshold 90

Your environment

• fusion-test-utils version: v1.0.2

• Node.js version (node --version): v8.10.0

• npm version (npm --version): v5.8.0

• Operating System: macOS High SIerra v10.13.3