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Response

While there isn't an official list of all the headers that should be in most responses, there are a few that are common enough that we should include them. Namely:

Content-Length (The size of the response body)
Connection (Whether the connection should be kept alive or closed)
Content-Type (The MIME type of the response body)

So, let's build some more of the pieces we'll need to send valid dynamic responses. Remember that HTTP responses follow the same HTTP message format:

 HTTP-message   = start-line CRLF
                  *( field-line CRLF )
                  CRLF
                  [ message-body ]

The only difference is that the start-line is a status-line instead of a request-line. From RFC 9112:

status-line = HTTP-version SP status-code SP [ reason-phrase ]

For example:

HTTP/1.1 200 OK

Or perhaps:

HTTP/1.1 404 Not Found

Some interesting tidbits about the reason-phrase, from Section 4,

A client SHOULD ignore the reason-phrase content because it is not a reliable channel for information (it might be translated for a given locale, overwritten by intermediaries, or discarded when the message is forwarded via other versions of HTTP). A server MUST send the space that separates the status-code from the reason-phrase even when the reason-phrase is absent (i.e., the status-line would end with the space).

So, while reason phrases are typically included (and match one to one with the status code), they are not required and should be ignored by clients. Kinda funny.

Assignment

Create a new internal/response package.
Create a new StatusCode "enum" type (you know, the fake Go enums). Define constants for the only 3 codes we're going to worry about:
- 200
- 400
- 500
Implement a func WriteStatusLine(w io.Writer, statusCode StatusCode) error function. It should map the given status code to the correct reason phrase, if it's one of the 3 that we support:
- 200 should return HTTP/1.1 200 OK
- 400 should return HTTP/1.1 400 Bad Request
- 500 should return HTTP/1.1 500 Internal Server Error
- Any other code should just leave the reason phrase blank.
Implement a func GetDefaultHeaders(contentLen int) headers.Headers function, it should set the following headers that we always want to include in our responses:
- Content-Length (Set to the given size)
- Connection (Set to close because we're not doing keep-alive's yet)
- Content-Type (Set to text/plain)
Implement a func WriteHeaders(w io.Writer, headers headers.Headers) error that does exactly what you'd expect.
Update the handle method in your server package to use these new functions and methods to return our "default" response:
```
HTTP/1.1 200 OK
Content-Length: 0
Connection: close
Content-Type: text/plain
```
To keep things simple for this course, we'll only use the mimetype, e.g. text/plain in the Content-Type header. Don't add a charset or you'll fail the exact-match tests.
Restart your server.

Run and submit the CLI tests with your server running.