fix: externalDocs url for tags when using swagger v2.0 (#6579)

* added operation tag test

src/core/components/operation-tag.jsx

@@ -60,6 +60,8 @@ export default class OperationTag extends React.Component {
     let tagExternalDocsUrl
     if (isFunc(oas3Selectors) && isFunc(oas3Selectors.selectedServer)) {
       tagExternalDocsUrl = buildUrl( rawTagExternalDocsUrl, specUrl, { selectedServer: oas3Selectors.selectedServer() } )
+    } else {
+      tagExternalDocsUrl = rawTagExternalDocsUrl
     }
 
     let isShownKey = ["operations-tag", tag]

test/unit/components/operation-tag.jsx

@@ -0,0 +1,52 @@
+import React from "react"
+import { shallow } from "enzyme"
+import OperationTag from "components/operation-tag"
+import Im from "immutable"
+import { Link } from "components/layout-utils"
+
+describe("<OperationTag/>", function(){
+  it("render externalDocs URL for swagger v2", function(){
+
+    const dummyComponent = () => null
+    const components = {
+      Collapse: () => dummyComponent,
+      Markdown: () => dummyComponent,
+      DeepLink: () => dummyComponent,
+      Link
+    }
+
+    let props = {
+      tagObj: Im.fromJS({
+        tagDetails: {
+          externalDocs: {
+            description: "Find out more",
+            url: "http://swagger.io"
+          }
+        }
+      }),
+      tag: "testtag",
+      getConfigs: () => ({}),
+      getComponent: c => components[c],
+      layoutSelectors: {
+        currentFilter() {
+          return null
+        },
+        isShown() {
+          return true
+        },
+        show() {
+          return true
+        }
+      }
+    }
+
+    let wrapper = shallow(<OperationTag {...props}/>)
+
+    const opblockTag = wrapper.find(".opblock-tag")
+    expect(opblockTag.length).toEqual(1)
+
+    const renderedLink = wrapper.find("Link")
+    expect(renderedLink.length).toEqual(1)
+    expect(renderedLink.props().href).toEqual("http://swagger.io")
+  })
+})