fix: FSET lowercasing upper case fields

[iwpnd]

Please ensure you adhere to every item in this list

Figured it is minor enough to not wait your okay on this @tidwall . :pray:

• [x] This PR was pre-approved by the project maintainer
• [x] I have self-reviewed the code
• [x] I have added all necessary tests

Describe your changes

moving the string.ToLower() conversion into the switch statement instead of mutating the input argument ensures field cases are preserved. added additional tests.

[...]
	for i := 3; i < len(args); i++ {
		arg := args[i]
		switch strings.ToLower(arg) {
		case "xx":
			xx = true
		default:
			fkey := arg
			i++
                        [...]

Issue number and link

fixes #741

[iwpnd]

An unrelated test appears to be failing. Looks to be flaky :eyes: