Move caser from a global variable to a local variable

[kojisaikiAtSony]

(This is split change from #339)

Problem

Parallel "Publish" request to a single goaws instance will randomly corrupt DataType values like below:

• S\u0000ring
• \u0000\u0000\u0000\u0000\u0000\u0000
• Sring\u0000
• S\u0000tingg

Cause

We should keep enclosing Caser instance in a request thread.
https://pkg.go.dev/golang.org/x/text@v0.20.0/cases#Caser

A Caser may be stateful and should therefore not be shared between goroutines.

There is a similar issue with this problem.
golang/go#59520

How reproduce and confirm the fix

We faced this problem with our business application but you can reproduce the same issue with the below code.

var caser = cases.Title(language.AmericanEnglish)

var dataType = []string{"String", "Binary"}

func TestCaser(t *testing.T) {
	for r := 0; r < 100000; r++ {
		var wg sync.WaitGroup
		for i := 0; i < len(dataType); i++ {
			name := dataType[i]
			wg.Add(1)
			go func() {
				fmt.Println(caser.String(name))
				wg.Done()
			}()
		}
		wg.Wait()
	}
}

You can confirm the fix in this PR works with this code.

var (
	caser = cases.Title(language.AmericanEnglish)
	mu    sync.Mutex
)

var dataType = []string{"String", "Binary"}

func formatTitle(input string) string {
	mu.Lock()
	defer mu.Unlock()
	return caser.String(input)
}

func TestCaser(t *testing.T) {
	for r := 0; r < 100000; r++ {
		var wg sync.WaitGroup
		for _, input := range dataType {
			wg.Add(1)
			go func(input string) {
				defer wg.Done()
				fmt.Println(formatTitle(input))
			}(input)
		}
		wg.Wait()
	}
}

[kojisaikiAtSony]

Hi @Admiral-Piett, we found the root cause of the strange behavior of caser we noted in #339. Could you please review this?

[Admiral-Piett]

@kojisaikiAtSony Awesome! Thank you so much for the detailed write up as well. That's SUPER helpful for me to figure out what I'm looking at. If you want to throw your nice mocked out test in there too for bonus, that would be cool, otherwise I'm ready to merge it as is. Up to you, so just let me know!

[kojisaikiAtSony]

I added the mocked out test but it's not passed CI yet... please wait 🙇

[kojisaikiAtSony]

Hi @Admiral-Piett , I added the test TestPublishRequest_SetAttributesFromForm_success_concurrent. If the caser instance moved to global again, the test may be failed with assertion error or panic randomly.