Make Query#to_cql handle invalid values more clearly

[eprothro]

type
#=> #<Cassandra::Types::Simple:0x007fed37c33300 @kind=:bigint>

params[i]
#=> nil

params
#=> ["55e05d24-513b-11e7-b6b8-33c23e6e0000", 2017-06-14 14:54:54 -0500, nil, nil, "55e0893e-513b-11e7-bd08-0f7a965973cc", "testuser94517816"]

bound_cql
#=> "INSERT INTO some_table (id, other_id, url, other_url, user_id, username) VALUES (?, ?, ?, ?, ?, ?);"

type
#<Cassandra::Types::Simple:0x007fed37c33300 @kind=:bigint>

type.new(nil)

     TypeError:
       can't convert nil into Integer

would be nice to catch type errors and give more info about what param is the culprit.