refactor: refactoring http request creation and sending (#395)

* refactoring http request creation and sending

* fix lint error

* increase the test coverage of client.go

* refactor: Change the style of HTTPRequestBuilder.Build func to one-argument-per-line.

completion.go

@@ -165,7 +165,7 @@ func (c *Client) CreateCompletion(
 		return
 	}
 
-	req, err := c.requestBuilder.Build(ctx, http.MethodPost, c.fullURL(urlSuffix, request.Model), request)
+	req, err := c.newRequest(ctx, http.MethodPost, c.fullURL(urlSuffix, request.Model), withBody(request))
 	if err != nil {
 		return
 	}