fix(OneOf): Fix oneof "NOT_SET" type incorrectly stripping underscores

[zakhenry]

Currently, if you have a oneof field that is labelled with an underscore in the type, the output typescript enum "NOT_SET" field strips the casing. This PR fixes that issue.

Proto:

    oneof two_words {
        bool a_2 = 3;
        bool b_2 = 4;
    }

Before:

    export enum TwoWordsCase {
        TWOWORDS_NOT_SET = 0,
    
    A_2 = 3,

    B_2 = 4,

    }

After:

    export enum TwoWordsCase {
        TWO_WORDS_NOT_SET = 0,
    
    A_2 = 3,

    B_2 = 4,

    }

Note the change is just a one liner, the huge diff is from adding a oneof to the example code.

[agreatfool]

Thank you for the great work. Just merged & new version 2.5.10 has been released.