Fix _timestamp in doc tables + arrays in advanced settings

[rashidkpc]

While testing a pull I noticed _timestamp was not accessible in Discover. There were 2 reasons for this:

• You could not add _timestamp to the metaFields array as array editing was broken. I broke out the logic for checking the type of a value for a setting and added tests for it.
• The metaFields list was not being checked when merging the fields key of a hit.

Closes #3318

[panda01]

I'm not entirely sure how to test this, I see metaFields in advanced settings, but don't know how it correlates to the discover screen.

Also, the settings that the user had there beforehand gets removed when this update is applied.

[rashidkpc]

Which setting is removed? Changes to metaFields? Thats probably a plus since if they'd changed it the value would be invalid as it would've been set to a string.

Testing the array editing part is easy:

1. on master, change metaFields, then pull the .kibana/config/4.0.2-snapshot document and look at the value. It will be a string. Remove the config doc
2. On the pr branch, change metaFields, then pull the .kibana/config/4.0.2-snapshot document and look at the value. It will be a array.

Testing the _timestamp part of this will require enabling _timestamp. You'll notice on master there's no way to get it to show up without the "No cached mapping for this field" error in the doc viewer.

[lukasolson]

LGTM!