Add form builder (#235)

* add form builder

* cover VariImage

* test for closing errors

* simplify tests

* add audio api test coverage

* don't leak authToken when printed

* rename api->client

* fix test

form_builder.go

@@ -0,0 +1,49 @@
+package openai
+
+import (
+	"io"
+	"mime/multipart"
+	"os"
+)
+
+type formBuilder interface {
+	createFormFile(fieldname string, file *os.File) error
+	writeField(fieldname, value string) error
+	close() error
+	formDataContentType() string
+}
+
+type defaultFormBuilder struct {
+	writer *multipart.Writer
+}
+
+func newFormBuilder(body io.Writer) *defaultFormBuilder {
+	return &defaultFormBuilder{
+		writer: multipart.NewWriter(body),
+	}
+}
+
+func (fb *defaultFormBuilder) createFormFile(fieldname string, file *os.File) error {
+	fieldWriter, err := fb.writer.CreateFormFile(fieldname, file.Name())
+	if err != nil {
+		return err
+	}
+
+	_, err = io.Copy(fieldWriter, file)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func (fb *defaultFormBuilder) writeField(fieldname, value string) error {
+	return fb.writer.WriteField(fieldname, value)
+}
+
+func (fb *defaultFormBuilder) close() error {
+	return fb.writer.Close()
+}
+
+func (fb *defaultFormBuilder) formDataContentType() string {
+	return fb.writer.FormDataContentType()
+}