Add operator= for vectors with different allocator

Now code: `vector<T> a(context); vector<T, OtherAlloc> b(context);
a = b;` works.

include/boost/compute/container/vector.hpp

@@ -252,6 +252,17 @@ public:
         return *this;
     }
 
+    template<class OtherAlloc>
+    vector& operator=(const vector<T, OtherAlloc> &other)
+    {
+        command_queue queue = default_queue();
+        resize(other.size(), queue);
+        ::boost::compute::copy(other.begin(), other.end(), begin(), queue);
+        queue.finish();
+
+        return *this;
+    }
+
     template<class OtherAlloc>
     vector& operator=(const std::vector<T, OtherAlloc> &vector)
     {