Previously, we were appending events to the event collection even when
the columns did not match!

Given the following three events:

```ruby
{
  "name": "my-series",
  "columns": [
    "cpu.user",
    "time"
  ],
  "points": [
    [
      5.0,
      1426288467
    ]
  ]
}
{
  "name": "my-series",
  "columns": [
    "memory.used",
    "time"
  ],
  "points": [
    [
      1000,
      1426288467
    ]
  ]
}
{
  "name": "my-series",
  "columns": [
    "cpu.user",
    "time"
  ],
  "points": [
    [
      6.0,
      1426288477
    ]
  ]
}
```

We would end up with the following batch:

```ruby
[
  {
    "name": "my-series",
    "columns": [
      "cpu.user",
      "time"
    ],
    "points": [
      [
        5.0,
        1426288467
      ],
      [
        6.0,
        1426288467
      ]
    ]
  },
  {
    "name": "my-series",
    "columns": [
      "memory.used",
      "time"
    ],
    "points": [
      [
        1000.0,
        1426288467
      ],
      [
        6.0, # this shouldn't be here!
        1426288467
      ]
    ]
  }
]