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

test/components/response-body.js

@@ -0,0 +1,36 @@
+import React from "react"
+import expect from "expect"
+import { shallow } from "enzyme"
+import ResponseBody from "components/response-body"
+import { inferSchema } from "corePlugins/samples/fn"
+
+describe("<ResponseBody />", function() {
+    const highlightCodeComponent = () => null
+    const components = {
+        highlightCode: highlightCodeComponent
+    }
+    const props = {
+        getComponent: c => components[c],
+    }
+
+    it("renders ResponseBody as 'application/json'", function() {
+        props.contentType = "application/json"
+        props.content = "{\"key\": \"a test value\"}"
+        const wrapper = shallow(<ResponseBody {...props}/>)
+        expect(wrapper.find("highlightCodeComponent").length).toEqual(1)
+    })
+
+    it("renders ResponseBody as 'text/html'", function() {
+        props.contentType = "application/json"
+        props.content = "<b>Result</b>"
+        const wrapper = shallow(<ResponseBody {...props}/>)
+        expect(wrapper.find("highlightCodeComponent").length).toEqual(1)
+    })
+
+    it("renders ResponseBody as 'image/svg'", function() {
+        props.contentType = "image/svg"
+        const wrapper = shallow(<ResponseBody {...props}/>)
+        console.warn(wrapper.debug())
+        expect(wrapper.find("highlightCodeComponent").length).toEqual(0)
+    })
+})