Add additional pool test & deprecate .end

There are no tests covering cursor.end().  It's also a weird method to have on the cursor as it terminates the connection to postgres internally.  I don't recall why I added this method in the first place but its not correct to close a connection to postgres by calling .end on a cursor...seems like a pretty big footgun.  If you have a pooled client and you call `cursor.end` it'll close the client internally and likely confuse the pool.  Plus it's just weird to be able to close a connection by calling .end on a query or cursor.  So...I'm deprecating that method.

index.js

@@ -154,13 +154,15 @@ Cursor.prototype._getRows = function(rows, cb) {
   this.connection.flush()
 }
 
-Cursor.prototype.end = function(cb) {
+// users really shouldn't be calling 'end' here and terminating a connection to postgres
+// via the low level connection.end api
+Cursor.prototype.end = util.deprecate(function(cb) {
   if (this.state !== 'initialized') {
     this.connection.sync()
   }
   this.connection.once('end', cb)
   this.connection.end()
-}
+}, 'Cursor.end is deprecated. Call end on the client itself to end a connection to the database.')
 
 Cursor.prototype.close = function(cb) {
   if (this.state === 'done') {

test/pool.js

@@ -83,4 +83,24 @@ describe('pool', function() {
         done()
       })
   })
+
+  it('can close multiple times on a pool', async function() {
+    const pool = new pg.Pool({ max: 1 })
+    const run = () =>
+      new Promise(async resolve => {
+        const cursor = new Cursor(text)
+        const client = await pool.connect()
+        client.query(cursor)
+        cursor.read(25, function(err) {
+          assert.ifError(err)
+          cursor.close(function(err) {
+            assert.ifError(err)
+            client.release()
+            resolve()
+          })
+        })
+      })
+    await Promise.all([run(), run(), run()])
+    await pool.end()
+  })
 })