Method GetFirst returns the first key found in a branch

[ulfox]

Describe the bug
Method GetFirst returns the first key that is found in the branch and not in the hierarchy.
This appears to be the case when the a branch that has the key is processed first.

Example:

key-1:
    key-2:
        key-3: value-1
    key-3:  value-2

Desired result for GetFirst("key-3") is value-2 but instead we get value-1

To Reproduce
Run GetFirst("someKey") with 2 keys

Expected behaviour
GetFirst should return value-1

[ulfox]

Fixed by: #8