fix: don't require siteUrl when host and sitemap are set (mdreizin#13)

* chore: format README.md

* fix: don't require siteUrl when host and sitemap are set

While the case of siteUrl not beeing present was handled, the code didn't
actually run because the graphql client emits an error when a non present field
is queried. This changes the behaviour so that a query is only executed when
either host or sitemap aren't set.

The change should be non-breaking.

README.md

@@ -95,6 +95,6 @@ module.exports = {
 };
 ```
 
-
 ## License
-[![FOSSA Status](https://app.fossa.io/api/projects/git%2Bgithub.com%2Fmdreizin%2Fgatsby-plugin-robots-txt.svg?type=large)](https://app.fossa.io/projects/git%2Bgithub.com%2Fmdreizin%2Fgatsby-plugin-robots-txt?ref=badge_large)
+
+[![FOSSA Status](https://app.fossa.io/api/projects/git%2Bgithub.com%2Fmdreizin%2Fgatsby-plugin-robots-txt.svg?type=large)](https://app.fossa.io/projects/git%2Bgithub.com%2Fmdreizin%2Fgatsby-plugin-robots-txt?ref=badge_large)

src/gatsby-node.js

@@ -36,20 +36,32 @@ function runQuery(handler, query) {
 }
 
 export async function onPostBuild({ graphql }, pluginOptions) {
-  const options = { ...pluginOptions };
+  const userOptions = { ...pluginOptions };
 
-  delete options.plugins;
+  delete userOptions.plugins;
 
-  const { site: { siteMetadata: { siteUrl } = {} } = {} } = await runQuery(
-    graphql,
-    query
-  );
   const defaultOptions = {
-    output: '/robots.txt',
-    host: siteUrl,
-    sitemap: siteUrl && url.resolve(siteUrl, 'sitemap.xml')
+    output: '/robots.txt'
   };
-  const { policy, sitemap, host, output } = { ...defaultOptions, ...options };
+
+  const mergedOptions = { ...defaultOptions, ...userOptions };
+
+  if (
+    !mergedOptions.hasOwnProperty('host') ||
+    !mergedOptions.hasOwnProperty('sitemap')
+  ) {
+    const {
+      site: {
+        siteMetadata: { siteUrl }
+      }
+    } = await runQuery(graphql, query);
+
+    mergedOptions.host = siteUrl;
+    mergedOptions.sitemap = url.resolve(siteUrl, 'sitemap.xml');
+  }
+
+  const { policy, sitemap, host, output } = mergedOptions;
+
   const content = await robotsTxt({
     policy,
     sitemap,