Based on conversation https://github.com/nock/nock/pull/1517/files#r280139478.

The `arguments.length <= 2` was confusing/misleading, and clearing the
`rawHeaders` clarifies the method's intention to anyone digging around,
plus reduces the changes for bugs in the future.

This commit only includes the implementation and tests.
Callers are to follow, but I didn’t want to muddy commit history.

Includes breaking changes.

- When a status code is provided followed by a function, the function returns the body. No magic.
- When a function is provided on its own, it _MUST_ return an array of `status, [body, [headers]]`. Again, no magic.

This change uses errors to enforce the following two signatures:
`.reply(status, [body, [headers]])` where `body` is any of the existing types that can result in a scalar value.
`.reply(() => [status, body = ‘’, headers = {}])` where the callback is called using the existing mechanics.

#### Breaking changes

- There is no longer a default value for the response status code. It must
be provided as the first arg of `reply` or the first element in the
returned array if using the single-function style.
  - `.reply()` will now throw an error

- Status code args will throw an error if they cannot be cast to ints.

- The single-function style callback **MUST** return an array with a
  length of 1-3 or else an error is thrown.

- If **NOT** using the single-function style callback, the resulting body
  will never be treated as a special value for status code or headers,
  it will always result to the responses body value.
  - `reply(200, () => [400, ‘foo’])`
    - **previous result** status: 400 body: ‘foo’
    - **new result** status: 200 body: “[400, ‘foo’]”
  - `reply(200, () => [400])`
    - **previous result** status: 400 body: “[400]”
    - **new result** status: 200 body: “[400]”

Calling `filteringPath` on the intercept instance was broken as the
transform fn set on the scope had the wrong name. Found when looking at
Uncovered lines in coveralls.

Prefer to not cast and instead have a hard requirement for the arg to already be an int.
Allow status code to not be provided and default to 200 again.

While `false` and `null` are falsy, they are valid JSON value so they
should be returned as a strings that JSON.parse would convert back to
native values.

Should fix nock#1446

`a` is not a local var.

Use different attribute names for the reply functions that return a body
vs return an array for the whole response (dynamic).

Rejigger RequestOverrider.end to return early based which flow is used.
It would be nice if this method was split up into more pure functions
to reduce the complexity.

This is a regression test for nock#1208.