Given sorted array of integers and target average implement a function which determine if there is a pair of values in the list where their average equals target average.
has_pair_average?([], 1.0) # false
has_pair_average?([3, 4, 7, 9], 6.5) # true
has_pair_average?([3, 4, 7, 9], 3.0) # false
has_pair_average?([3, 5, 9, 7, 11, 14], 8.0) # true