self.encoding ignore both undefined and null

[wangxiaxin]

when using with pbjs , original value of self.encoding is undefined,we wish to keep response.body Buffer style

PR Checklist:

• I have run npm test locally and all tests are passing.
• I have added/updated tests for any new behavior.
  
• If this is a significant change, an issue has already been created where the problem / solution was discussed: [N/A, or add link to issue here]
  

PR Description

when using with pbjs , original value of self.encoding is undefined,we wish to keep response.body Buffer style

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.