title: Map List<String> with Mapstruct from Java POJO to Protobuf (proto3)
body: <p>I'm trying to map some POJOs from Java to Protobuf (proto3). Some of them contain Lists. While mapping lists with POJOs (for example List) is no problem, I'm getting a UnsupportedOperationException. </p>
  
  <p>Example with <code>List&lt;Product&gt;</code> (this works corrctly):</p>
  
  <pre><code>ProductProtobuf.Builder map(Product product);
  
  @Mapping(target = "productsList", source = "products")
  ResponseProtobuf.Builder map(Response response);
  </code></pre>
  
  <p>Example with <code>List&lt;String&gt;</code> (this doesn't work):</p>
  
  <pre><code>@Mapping(target = "usersList", source = "users")
  ResponseProtobuf.Builder map(Response response);
  </code></pre>
  
  <p>Additionally, I have some Mapper for builder:</p>
  
  <pre><code>public ResponseProtobuf.Builder responseBuilder() {
      return ResponseProtobuf.newBuilder();
  }
  
  public ProductProtobuf build(ProductProtobuf.Builder builder) {
      return builder.build();
  }
  </code></pre>