[Backport] Fix proxy generation return type

[jignesh-baldha]

Original Pull Request

#17552
This PR attempts to fix Proxy generation when return type is defined in class.

Description

Code generator for Proxy is missing returnType in the method info definition. Another neighbour generator classes already have this defined, but it seems it was forgotten here.

Fixed Issues (if relevant)

I've been looking through the issues but it seems nobody has reported this bug.

Manual testing scenarios

1. Use provided Gist to create a file in the project root folder: https://gist.github.com/adrian-martinez-interactiv4/38bca4531085093f3c99553ff936d478
2. Execute it, ensuring you delete generated/code folder each time you are executing it.

Actual output

Expected output

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[ihor-sviziev]

Magneto 2.1.x can be ran on php 5.6.x, that don't have return types yet. So we can't accept this PR. I'm closing it