Add response headers option support + add HEAD HTTP request method mocking support

[NeilujD]

Some informations I need to mock are only existing in the response headers.
I thus implemented a simple way to support that in this addon.

Here is how it should work:

...
mockData: [
  {
    url: "/my-url",
    method: 'HEAD',
    status: 200,
    responseHeaders: {
      'X-Cache-Status': 'HIT'
    },
    response: {}
  }
]
...

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