fix: separate a trailing capital in a default env name

[SashaMIT]

Summary

UseFieldNameByDefault builds the env name in toEnvName. A capital that ends the field was not split from the lowercase letter before it, because that check only ran when another character followed. MyA became MYA, so MY_A=set left the field empty. FooBar was already FOO_BAR. HTTP and userID stay HTTP and USER_ID.

Test plan

• toEnvName("MyA") was MYA before the change and MY_A after
• ParseWithOptions with UseFieldNameByDefault and MY_A=set left MyA empty before the change and reads set after
• go test ./...