Add support for 'format' tag in response headers

[tompson-wu]

We need to describe response header type for long/int64

I write code like:

// @Header 200 integer Content-Length "the content length" Format(int64)

But the result is:

"Content-Length": {
    "type": "integer",
    "description": "the content length\" Format(int64)"
}

This result is expected:

"Content-Length": {
    "type": "integer",
    "description": "the content length",
    "format": "int64"
}

[ubogdan]

Headers annotation doesn't support the "format" at this time. Any contribution is welcome.