LibWeb: Implement XMLHttpRequest.overrideMimeType

This allows you to ignore the Content-Type returned by the server and
always parse the content as if it's the given MIME type.

This will currently be used for allowing you to override the charset
of text responses.

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp

@@ -291,4 +291,21 @@ String XMLHttpRequest::get_all_response_headers() const
     return builder.to_string();
 }
 
+// https://xhr.spec.whatwg.org/#dom-xmlhttprequest-overridemimetype
+DOM::ExceptionOr<void> XMLHttpRequest::override_mime_type(String const& mime)
+{
+    // 1. If this’s state is loading or done, then throw an "InvalidStateError" DOMException.
+    if (m_ready_state == ReadyState::Loading || m_ready_state == ReadyState::Done)
+        return DOM::InvalidStateError::create("Cannot override MIME type when state is Loading or Done.");
+
+    // 2. Set this’s override MIME type to the result of parsing mime.
+    m_override_mime_type = MimeSniff::MimeType::from_string(mime);
+
+    // 3. If this’s override MIME type is failure, then set this’s override MIME type to application/octet-stream.
+    if (!m_override_mime_type.has_value())
+        m_override_mime_type = MimeSniff::MimeType("application"sv, "octet-stream"sv);
+
+    return {};
+}
+
 }