fix issue when site returns single result

src/oai-pmh-list.js

@@ -29,7 +29,7 @@ export async function * getOaiListItems (oaiPmh, verb, field, options) {
   })
   const initialParsedResponse = await parseOaiPmhXml(initialResponse.body)
   const initialResult = initialParsedResponse[verb]
-  for (const item of initialResult[field]) {
+  for (const item of [].concat(initialResult[field])) {
     yield item
   }
 
@@ -45,7 +45,7 @@ export async function * getOaiListItems (oaiPmh, verb, field, options) {
     })
     const parsedResponse = await parseOaiPmhXml(response.body)
     result = parsedResponse[verb]
-    for (const item of result[field]) {
+    for (const item of [].concat(result[field])) {
       yield item
     }
   }

src/oai-pmh.test.js

@@ -189,5 +189,17 @@ describe('OaiPmh', () => {
         { setSpec: 'stat', setName: 'Statistics' }
       ])
     })
+
+    it('should list arxiv sets single result', async () => {
+      const oaiPmh = new OaiPmh(arxivBaseUrl)
+      const res = []
+      for await (const set of oaiPmh.listSets()) {
+        res.push(set)
+      }
+      res.should.containDeep([
+        { setSpec: 'cs', setName: 'Computer Science' }
+      ])
+      res.should.have.length(1)
+    })
   })
 })

test/nock-fixtures/oaipmh-listsets---should-list-arxiv-sets-single-result.json

@@ -0,0 +1,24 @@
+[
+    {
+        "scope": "http://export.arxiv.org:80",
+        "method": "GET",
+        "path": "/oai2?verb=ListSets",
+        "body": "",
+        "status": 200,
+        "response": "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<OAI-PMH xmlns=\"http://www.openarchives.org/OAI/2.0/\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xsi:schemaLocation=\"http://www.openarchives.org/OAI/2.0/ http://www.openarchives.org/OAI/2.0/OAI-PMH.xsd\">\n<responseDate>2018-06-21T14:31:06Z</responseDate>\n<request verb=\"ListSets\">http://export.arxiv.org/oai2</request>\n<ListSets>\n<set>\n <setSpec>cs</setSpec>\n <setName>Computer Science</setName>\n</set>\n</ListSets>\n</OAI-PMH>\n",
+        "rawHeaders": [
+            "Date",
+            "Thu, 21 Jun 2018 14:31:05 GMT",
+            "Server",
+            "Apache",
+            "Vary",
+            "Accept-Encoding,User-Agent",
+            "Connection",
+            "close",
+            "Transfer-Encoding",
+            "chunked",
+            "Content-Type",
+            "text/xml"
+        ]
+    }
+]