Fix merge operator to handle onCompleted of input observables not a destructive way

The merge operator removed the source observable if it complete.
After that the subscriptions after that source observable index can not unsubscribe.
Fix the issue by recording the completed source index, as combineLatest operator does.

rx.lua

@@ -1049,6 +1049,7 @@ function Observable:merge(...)
   table.insert(sources, 1, self)
 
   return Observable.create(function(observer)
+    local completed = {}
     local subscriptions = {}
 
     local function onNext(...)
@@ -1061,9 +1062,9 @@ function Observable:merge(...)
 
     local function onCompleted(i)
       return function()
-        sources[i] = nil
+        table.insert(completed, i)
 
-        if not next(sources) then
+        if #completed == #sources then
           observer:onCompleted()
         end
       end

src/operators/merge.lua

@@ -9,6 +9,7 @@ function Observable:merge(...)
   table.insert(sources, 1, self)
 
   return Observable.create(function(observer)
+    local completed = {}
     local subscriptions = {}
 
     local function onNext(...)
@@ -21,9 +22,9 @@ function Observable:merge(...)
 
     local function onCompleted(i)
       return function()
-        sources[i] = nil
+        table.insert(completed, i)
 
-        if not next(sources) then
+        if #completed == #sources then
           observer:onCompleted()
         end
       end

tests/merge.lua

@@ -20,6 +20,20 @@ describe('merge', function()
     expect(#unsubscribeB).to.equal(1)
   end)
 
+  it('unsubscribes from all input observables included completed', function()
+    local observableA = Rx.Observable.empty()
+
+    local unsubscribeB = spy()
+    local subscriptionB = Rx.Subscription.create(unsubscribeB)
+    local observableB = Rx.Observable.create(function(observer)
+      return subscriptionB
+    end)
+
+    local subscription = observableA:merge(Rx.Observable.empty(), observableB):subscribe()
+    subscription:unsubscribe()
+    expect(#unsubscribeB).to.equal(1)
+  end)
+
   it('produces values from all input observables, in order', function()
     local observableA = Rx.Subject.create()
     local observableB = Rx.Subject.create()