fix: incorrect usage of lodash.lowerCase (via #4200)

* Wrong usage of https://lodash.com/docs/4.17.5#lowerCase

* Add basic tests about ResponseBody Content-Type

src/core/components/response-body.jsx

@@ -1,7 +1,7 @@
 import React from "react"
 import PropTypes from "prop-types"
 import formatXml from "xml-but-prettier"
-import lowerCase from "lodash/lowerCase"
+import toLower from "lodash/toLower"
 import { extractFileNameFromContentDispositionHeader } from "core/utils"
 import win from "core/window"
 
@@ -110,7 +110,7 @@ export default class ResponseBody extends React.PureComponent {
       bodyEl = <HighlightCode downloadable fileName={`${downloadName}.xml`} value={ body } />
 
       // HTML or Plain Text
-    } else if (lowerCase(contentType) === "text/html" || /text\/plain/.test(contentType)) {
+    } else if (toLower(contentType) === "text/html" || /text\/plain/.test(contentType)) {
       bodyEl = <HighlightCode downloadable fileName={`${downloadName}.html`} value={ content } />
 
       // Image