Bugfix2

[mohammadJaliliTorkamani]

added assertion message to two of the assertion:
assert (now == 0)
and
assert (result.length() == 20)

located at src/main/java/com/google/firebase/database/utilities/PushIdGenerator.java

to make program debugging more efficient.
As well, a new assertion was added to check the value of the variable "lastRandChars" as follows:

assert (lastRandChars[i] >= 0 && lastRandChars[i] < 64) : "Number generated by Random should be between 0 and 63";

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