Fix bug handling HttpResponseMessage that has no content

[zachabney]

Fixes bug where an error is thrown when the server tries to handle a HttpResponseMessage that does not contain any message content, as is the case when using BaseHttpController helper functions.

Description

Moved code handling content headers inside undefined conditional check.

Related Issue

inversify/InversifyJS#912

Motivation and Context

Fixes bug where server would throw an error with a response that has no content.

How Has This Been Tested?

Tested InversifyExpressServer#handleHttpResponseMessage to ensure the express.Response#sendStatus would be called with a HttpResponseMessage that has no content.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.