fixed panic on fetch returning empty response

[syumai]

What

• Stopped using ToStreamResponse in fetch func and replaced it to ToResponse.

Motivation

• Current implementation of fetch panics when fetch result body is empty (No Content).
  • Error with Fetch returning 204 No Content response #122
• The piping logic to TransformStream is causing this issue.
  • Stream responses without TransformStream piping are already working since the following commit: bf3ab8e.
  • Therefore, I've stopped using TransformStream piping.