Properly fill out the w3c unexpected alert expcetion alert text

As described in https://w3c.github.io/webdriver/webdriver-spec.html#dfn-handle-any-user-prompts

When returning an error with unexpected alert open, a remote end
may return the current text (as if returned by Get Alert Text)
of the prompt as the value of the text key in the JSON Object
that is the value of the data key in the error response, as
shown in this non-normative example:

```{
    error: "unexpected alert open",
    message: "implementation defined",
    data: {
        text: "the text from the alert"
    }
}```

Author: shs96c

java/client/src/org/openqa/selenium/remote/http/W3CHttpResponseCodec.java

@@ -19,13 +19,15 @@
 
 import static com.google.common.base.Strings.nullToEmpty;
 import static com.google.common.net.HttpHeaders.CONTENT_TYPE;
+import static java.net.HttpURLConnection.HTTP_INTERNAL_ERROR;
 import static java.net.HttpURLConnection.HTTP_OK;
 
 import com.google.common.base.Strings;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
 import com.google.gson.JsonParser;
 
+import org.openqa.selenium.UnhandledAlertException;
 import org.openqa.selenium.WebDriverException;
 import org.openqa.selenium.remote.ErrorCodes;
 import org.openqa.selenium.remote.JsonToBeanConverter;
@@ -96,7 +98,22 @@ public Response decode(HttpResponse encodedResponse) {
 
       response.setState(error);
       response.setStatus(errorCodes.toStatus(error, Optional.of(encodedResponse.getStatus())));
-      response.setValue(createException(error, message));
+
+      // For now, we'll inelegantly special case unhandled alerts.
+      if ("unexpected alert open".equals(error) &&
+          HTTP_INTERNAL_ERROR == encodedResponse.getStatus()) {
+        String text = "";
+        JsonElement data = obj.get("data");
+        if (data != null) {
+          JsonElement rawText = data.getAsJsonObject().get("text");
+          if (rawText != null) {
+            text = rawText.getAsString();
+          }
+        }
+        response.setValue(new UnhandledAlertException(message, text));
+      } else {
+        response.setValue(createException(error, message));
+      }
       return response;
     }
 

java/client/test/org/openqa/selenium/remote/http/W3CHttpResponseCodecTest.java

@@ -24,14 +24,17 @@
 import static org.junit.Assert.assertTrue;
 import static org.openqa.selenium.remote.ErrorCodes.METHOD_NOT_ALLOWED;
 
+import com.google.common.collect.ImmutableMap;
 import com.google.gson.Gson;
 
 import org.junit.Test;
+import org.openqa.selenium.UnhandledAlertException;
 import org.openqa.selenium.UnsupportedCommandException;
 import org.openqa.selenium.WebDriverException;
 import org.openqa.selenium.remote.ErrorCodes;
 import org.openqa.selenium.remote.Response;
 
+import java.io.Serializable;
 import java.util.HashMap;
 import java.util.Map;
 
@@ -89,6 +92,22 @@ public void decodingAnErrorWithoutAStacktraceIsDecodedProperlyForConformingImple
     assertTrue(((WebDriverException) decoded.getValue()).getMessage().contains("I like peas"));
   }
 
+  @Test
+  public void shouldPopulateTheAlertTextIfThrowingAnUnhandledAlertException() {
+    ImmutableMap<String, ImmutableMap<String, Serializable>> data = ImmutableMap.of(
+        "value", ImmutableMap.of(
+            "error", "unexpected alert open",
+            "message", "Modal dialog present",
+            "stacktrace", "",
+            "data", ImmutableMap.of("text", "cheese")));
+
+    HttpResponse response = createValidResponse(500, data);
+    Response decoded = new W3CHttpResponseCodec().decode(response);
+
+    UnhandledAlertException ex = (UnhandledAlertException) decoded.getValue();
+    assertEquals("cheese", ex.getAlertText());
+  }
+
   private HttpResponse createValidResponse(int statusCode, Map<String, ?> data) {
     byte[] contents = new Gson().toJson(data).getBytes(UTF_8);