ReadCloser should able to read content directly, or add function to access content, instead of using io.ReadAll

[wondar-chan]

package: /gogf/gf/[email protected]/internal/utils/utils_io.go:20
suggestion: why not export a function, called GetContent eg. so we can direct access ReadCloser.content. we all know, io.ReadAll behave bad when data is too big. With a funciton which can aceess content, we no need to use io.ReadAll, so that we can reduce memery usage.