Downgrade to go1.21, Request wrapper

[glossd]

Fixes #33
Because I created fetch.Request with embedded http.Request, my library no longer needs to the PathValue() method, allowing me to downgrade to go1.21
Migration guide:
If you have ever discovered fetch.Request function before, replace it with fetch.Do
If you used json tags in the input of ToHandlerFunc, wrap the input with fetch.Request and access the needed data.
I removed DuplicateHeaders field from everywhere, just rely on Headers