Stop calling WriteHeader in Write (#15862)

Fixes http: superfluous response.WriteHeader call from code.gitea.io/gitea/modules/context.(*Response).WriteHeader (response.go:67)

* Looking again we don't need this writeHeader as all of our downstream
implementations will always do it for us

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>

modules/context/response.go

@@ -49,7 +49,7 @@ func (r *Response) Write(bs []byte) (int, error) {
 		return size, err
 	}
 	if r.status == 0 {
-		r.WriteHeader(200)
+		r.status = http.StatusOK
 	}
 	return size, nil
 }