a fix for failure to compile with OpenJDK11

[ostueker]

The current version of the library fails to compile with OpenJDK11 with the following error:

$ JAVA_HOME=/usr/lib/jvm/java-1.11.0-openjdk-amd64  mvn clean compile
[...]
[INFO] -------------------------------------------------------------
[ERROR] COMPILATION ERROR :
[INFO] -------------------------------------------------------------
[ERROR] /home/ostueker/software-and-algorithms/src/main/java/blogspot/software_and_algorithms/stern_library/data_structure/ThriftyList.java:[1030,12] reference to toArray is ambiguous
  both method <T>toArray(java.util.function.IntFunction<T[]>) in java.util.Collection and method <U>toArray(U[]) in blogspot.software_and_algorithms.stern_library.data_structure.ThriftyList match
[INFO] 1 error
[INFO] -------------------------------------------------------------
[...]

This patch seems to fix the issue, though I don't understand the code nearly well enough to judge whether my solution does the right thing. The tests however, still pass after applying the patch.