groupby casts string column to int

[td7650]

Describe the bug
Running the example code below results in wrong column type

To Reproduce
Steps to reproduce the behavior:

1. Open the attached test.html in your browser
2. or group a string column with numerical values like "2021-01" and count another column

var json_data = [
  { key: "P1", month: "2021-01", storypoints: 3 },
  { key: "P2", month: "2021-02", storypoints: 1 },
  { key: "P3", month: "2021-02", storypoints: 3 },
  { key: "P4", month: "2021-03", storypoints: 2 },
  { key: "P5", month: "2021-04", storypoints: 3 }
];
let df = new dfd.DataFrame(json_data);
df.plot("div1").table()

let grp = df.groupby(["month"]);
let new_df = grp.col(["key"]).count();
new_df.plot("div2").table()

Expected behavior

1. column "month" in grouped dataframe should show values like "2021-01", "2021-02", ...
2. instead the whole columns datatype is changed from string to int and it shows multiple entries of "2021", "2021"
3. the month part after the "-" is cut off

Screenshots
test.zip - zipped test.html

Desktop (please complete the following information):

• OS: Win 10
• Browser chrome
• Version 89.0.4389.128