use contextual delim-precedes-last for Amer Sociol Assoc

This fixed our support issue as follows:

In ASA, sources with **two authors** currently have an extra comma after the first author's first name. The comma after the first name should be removed (p. 46 in ASA guide).

**Current Output:**
Kirchenbaum, Michele, and Caity Selleck. 2014. *The Greatest Book About Libraries Ever*. New York, NY: Pearson.

**Expected Output:**
Kirchenbaum, Michele and Caity Selleck. 2014. *The Greatest Book About Libraries Ever*. New York, NY: Pearson.

Note: This does not apply to citations with 3+ authors.