Parse HTML GET command

I don't think i can use a HTTP library since the arduino acts like a webserver, and the HTTP library i found was a client. (GitHub - amcewen/HttpClient: Arduino HTTP library)

But I've changed the HTML code to use POST instead of GET, but now the data I receive on the serial monitor is POST / HTTP/1.1

What is going wrong here?

My code is

#include <SPI.h>
#include <Ethernet.h>

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

// MAC address from Ethernet shield sticker under board
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };


IPAddress ip(192, 168, 0, 20);   // IP address, may need to change depending on network
EthernetServer server(80);       // create a server at port 80

String readString;

LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7); // Set the LCD I2C address

void setup()
{
  // disable Ethernet chip
  pinMode(10, OUTPUT);
  digitalWrite(10, HIGH);
  pinMode(4, OUTPUT);

  Serial.begin(115200);       // for debugging

  lcd.begin(16, 2);
  lcd.setBacklightPin(3, POSITIVE);
  lcd.setBacklight(HIGH);
  lcd.clear();
  // Message on LCD
  lcd.setCursor(2, 0);
  lcd.print("Initializing");
  Serial.println("Initializing...");

  // initialize SD card

  if (Ethernet.begin(mac) == 0) {
    lcd.clear();
    lcd.setCursor(2, 0);
    lcd.print(F("DHCP Failed!"));
    lcd.setCursor(5, 1);
    lcd.print(F("Halted"));
    Serial.println("DHCP Failed!");
    Serial.println("Application halted");
    LoopForever();
  }
  server.begin();           // start to listen for clients

  // End of initialization message on LCD + print IP address
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print(F("IP:"));
  lcd.setCursor(0, 1);
  lcd.print(Ethernet.localIP());
  Serial.print("Current IP: ");
  Serial.println(Ethernet.localIP());
}

void loop()
{
  // Create a client connection
  EthernetClient client = server.available();
  if (client) {
    while (client.connected()) {
      if (client.available()) {
        char c = client.read();

        //read char by char HTTP request
        if (readString.length() < 100) {

          //store characters to string
          readString += c;
          //Serial.print(c);
        }

        //if HTTP request has ended
        if (c == '\n') {

          ///////////////
          Serial.println(readString); //print to serial monitor for debuging

          //now output HTML data header
          if (readString.indexOf('?') >= 0) { //don't send new page
            client.println("HTTP/1.1 204 Sander");
            client.println();
            client.println();
          }
          else {
            client.println("HTTP/1.1 200 OK"); //send new page
            client.println("Content-Type: text/html");
            client.println();

            client.println("<HTML>");
            client.println("<HEAD>");
            client.println("<title>Network billboard</title>");
            client.println("</HEAD>");
            client.println("<BODY>");

            client.println("<H1>Network billboard</H1>");

            // Dit gedeelte HTML code is zo omgezet dat het wel meegecompileerd kan worden in C
            // Gebruikte website: http://tomeko.net/online_tools/cpp_text_escape.php?lang=en

            client.println("<form action=\"\" method=\"post\">"); //"<form action="" method="post">"
            client.println("First line: <input type=\"text\" name=\"fline\" maxlength=\"50\" autocomplete=\"off\" autocorrect=\"off\">
"); //"First line: <input type="text" name="fline" maxlength="50" autocomplete="off" autocorrect="off">
"

            client.println("Second line: <input type=\"text\" name=\"sline\" maxlength=\"50\" autocomplete=\"off\" autocorrect=\"off\">
"); //"Second line: <input type="text" name="sline" maxlength="50" autocomplete="off" autocorrect="off">
"
            client.println("<input type=\"submit\" value=\"Submit\">"); //<input type="submit" value="Submit">
            client.println("</form>");

            client.println("</BODY>");
            client.println("</HTML>");
          }

          delay(1);
          //stopping client
          client.stop();

          Serial.println(readString);

          // Parse incoming data code goes here //

          /////////////////////////////////////////////         

          //clearing string for next read
          readString = "";

        }
      }
    }
  }
}

void LoopForever()
{
  // Looping forever here
  LoopForever();
}