Fix case of Month token in datetime to string conversion

Currently Hive doesn't allow month tokens with weird spelling like 'MONth', 'mONTH' etc. However, Oracle does and Hive should follow that approach.

The rules:

• If the first letter is lowercase then the output is lowercase: 'mONTH' -> 'may'
• If the first two letters are uppercase then the output is uppercase: 'MOnth' -> 'MAY'
• If the first letter is uppercase and the second is lowercase then the output is capitalized: 'Month' -> 'May'.

Oracle:

select to_char(to_timestamp('2019-05-10', 'YYYY-MM-DD'), 'MOnthYYYY') from DUAL;
MAY      2019

select to_char(to_timestamp('2019-05-10', 'YYYY-MM-DD'), 'mONTHYYYY') from DUAL;
may      2019

select to_char(to_timestamp('2019-05-10', 'YYYY-MM-DD'), 'MoNTHYYYY') from DUAL;
May      2019

Please check the same for 'Name of the day' tokens.