Return reference from BoundedVector::emplace_back (#730)

Using `auto` made `BoundedVector::emplace_back()` return a copy instead
of a reference. Code such as the following would not compile in that
case:
```
auto & value = bounded_vector.emplace_back();
```
Due to the `auto` usage an rvalue was returned which couldn't be bound
to an lvalue reference.

Signed-off-by: Alexander Hans <[email protected]>

Author: ahans

rosidl_runtime_cpp/include/rosidl_runtime_cpp/bounded_vector.hpp

@@ -474,7 +474,7 @@ class BoundedVector
    * \param args Arguments to be forwarded to the constructor of Tp
    */
   template<typename ... Args>
-  auto
+  typename Base::reference
   emplace_back(Args && ... args)
   {
     if (size() >= UpperBound) {

rosidl_runtime_cpp/test/test_bounded_vector.cpp

@@ -43,7 +43,8 @@ TEST(rosidl_generator_cpp, bounded_vector_rvalue) {
   ASSERT_EQ(v.size(), 0u);
   ASSERT_EQ(v.max_size(), 2u);
   v.emplace_back(1);
-  v.emplace_back(2);
+  auto & value = v.emplace_back(2);
+  ASSERT_EQ(value, 2);
   ASSERT_THROW(v.emplace_back(3), std::length_error);
   ASSERT_EQ(v.size(), 2u);
   // move assignment