Fix handling of arguments with wrong type in snapshot and string methods.

[dinoboff]

• fix snapshot methods.
• fix string methods.

Fix #86

[dinoboff]

Actually, snapshot behave the same than string methods:

• root.child("foo").child(auth.foo).exists() with auth.foo == null evaluate to false
• root.child("foo").child(auth.foo).exists() == false with auth.foo == null evaluate to false

I will fix the fix to throw when the snapshot methods receive anything else than strings.