Write a Python function `sum_of_common_divisors(a, b)` that calculates the sum of all common divisors of two given positive integers `a` and `b`.
A divisor of a number is an integer that divides the number without leaving a remainder. For example, the divisors of 10 are 1, 2, 5, and 10.
#### Example Usage
```python [main.nopy]
print(sum_of_common_divisors(10, 15)) # Output: 6 (common divisors: 1, 5)
print(sum_of_common_divisors(100, 150)) # Output: 93 (common divisors: 1, 2, 5, 10, 25, 50)
print(sum_of_common_divisors(4, 6)) # Output: 3 (common divisors: 1, 2)
```