check isUpsert header in POST document request (#5)

* check isUpsert header in POST document request

* Verify response code on "CreateItem that already exists" test

---------

Co-authored-by: Pijus Kamandulis <pikami@users.noreply.github.com>

api/handlers/documents.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/http"
+	"strconv"
 
 	jsonpatch "github.com/evanphx/json-patch/v5"
 	"github.com/gin-gonic/gin"
@@ -223,6 +224,11 @@ func DocumentsPost(c *gin.Context) {
 		return
 	}
 
+	isUpsert, _ := strconv.ParseBool(c.GetHeader("x-ms-documentdb-is-upsert"))
+	if isUpsert {
+		repositories.DeleteDocument(databaseId, collectionId, requestBody["id"].(string))
+	}
+
 	createdDocument, status := repositories.CreateDocument(databaseId, collectionId, requestBody)
 	if status == repositorymodels.Conflict {
 		c.IndentedJSON(http.StatusConflict, gin.H{"message": "Conflict"})