fix Excess found: excess = 2 url = / (zero-length body) in HEAD response

[mmatczuk]

We need e2e HEAD test:

• with and without body
• Transfer-Encoding: (none just content-length), chunked, identity

[what-the-diff]

PR Summary

• Error Handling Enhanced in Callback Setup
  If there are any issues during execution, these will now be reported more accurately. This was achieved by changing the code to return an error if the error outputs (stderr) is not empty.

• Improvements to Handler.go in Terms of Handling Empty Body
  There were some adjustments made to correctly handle cases where the body is http.NoBody. This ensures a more stable performance even when the body data is missing.

• Addition of head.go
  A new file named head.go has been introduced to the code. Although the details of what this new file does weren't shared, its addition will likely provide some great new functionality.

• TEST Upgrades in httpspec_test.go
  New testing measures have been inserted to ensure the proper operation of our services. To be specific, the Transfer-Encoding header was added to the request and response headers in our tests.

• Modification in Proxy API Responses
  Changes were made to manually write the response for HEAD requests with no body, ensuring an optimal response handling. Server-Sent Events (SSE) will now be treated more efficiently by flushing after each chunk.

[Choraden]

LGTM