LibHTTP: Implement getting the correct reason phrase from HttpResponse

This adds a reason_phrase() getter and a static reason_phrase_for_code()
to the HttpResponse class.

It also changes the class to use east const style.

Userland/Libraries/LibHTTP/HttpResponse.cpp

@@ -18,4 +18,63 @@ HttpResponse::~HttpResponse()
 {
 }
 
+StringView HttpResponse::reason_phrase_for_code(int code)
+{
+    VERIFY(code >= 100 && code <= 599);
+
+    static HashMap<int, StringView> s_reason_phrases = {
+        { 100, "Continue"sv },
+        { 101, "Switching Protocols"sv },
+        { 200, "OK"sv },
+        { 201, "Created"sv },
+        { 202, "Accepted"sv },
+        { 203, "Non-Authoritative Information"sv },
+        { 204, "No Content"sv },
+        { 205, "Reset Content"sv },
+        { 206, "Partial Content"sv },
+        { 300, "Multiple Choices"sv },
+        { 301, "Moved Permanently"sv },
+        { 302, "Found"sv },
+        { 303, "See Other"sv },
+        { 304, "Not Modified"sv },
+        { 305, "Use Proxy"sv },
+        { 307, "Temporary Redirect"sv },
+        { 400, "Bad Request"sv },
+        { 401, "Unauthorized"sv },
+        { 402, "Payment Required"sv },
+        { 403, "Forbidden"sv },
+        { 404, "Not Found"sv },
+        { 405, "Method Not Allowed"sv },
+        { 406, "Not Acceptable"sv },
+        { 407, "Proxy Authentication Required"sv },
+        { 408, "Request Timeout"sv },
+        { 409, "Conflict"sv },
+        { 410, "Gone"sv },
+        { 411, "Length Required"sv },
+        { 412, "Precondition Failed"sv },
+        { 413, "Payload Too Large"sv },
+        { 414, "URI Too Long"sv },
+        { 415, "Unsupported Media Type"sv },
+        { 416, "Range Not Satisfiable"sv },
+        { 417, "Expectation Failed"sv },
+        { 426, "Upgrade Required"sv },
+        { 500, "Internal Server Error"sv },
+        { 501, "Not Implemented"sv },
+        { 502, "Bad Gateway"sv },
+        { 503, "Service Unavailable"sv },
+        { 504, "Gateway Timeout"sv },
+        { 505, "HTTP Version Not Supported"sv }
+    };
+
+    if (s_reason_phrases.contains(code))
+        return s_reason_phrases.ensure(code);
+
+    // NOTE: "A client MUST understand the class of any status code, as indicated by the first
+    //       digit, and treat an unrecognized status code as being equivalent to the x00 status
+    //       code of that class." (RFC 7231, section 6)
+    auto generic_code = (code / 100) * 100;
+    VERIFY(s_reason_phrases.contains(generic_code));
+    return s_reason_phrases.ensure(generic_code);
+}
+
 }