Update HashMap.java

The changes are pretty insignificant. added toStrings and Iterator.
The most important change here is the difference between currentSize-the count of key to value mappings
and tableSize-the table size; both global variables

Also in constructor declare the hashing function: 
public HashMap(HashingFunction<KeyType> hf, double maxLoad) {
        hashFunc = hf;
        ......
        }
    }