algs4partII-004/boggle: Add TrieSET::hasPrefix method.

Compilation:  PASSED
Style:        FAILED
Findbugs:     No potential bugs found.
API:          PASSED

Correctness:  13/13 tests passed
Memory:       3/3 tests passed
Timing:       6/6 tests passed

Aggregate score: 100.00% [Correctness: 65%, Memory: 10%, Timing: 25%, Style: 0%]