Override keys are case sensitive

[jakelazaroff]

Using weird casing in overridden elements doesn't replace them. Possibly related to #247

Example:

import React from "react";
import ReactDOM from "react-dom";
import Markdown from "markdown-to-jsx";

function CustomInput() {
  return <div>custom input would go here</div>;
}

function App() {
  return (
    <Markdown options={{ overrides: { input: CustomInput } }}>
      {"<INPUT />"}
    </Markdown>
  );
}

const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);