Term Vectors: Fix NPE with dfs and no tvs

Fixes a bug with dfs option for when term vectors are not stored and not generated.
Provide more context variables in update scripts

In addition to `_source`, the following variables are available through
the `ctx` map: `_index`, `_type`, `_id`, `_version`, `_routing`,
`_parent`, `_timestamp`, `_ttl`.

Some of these fields are more useful still within the context of an
Update By Query, see #1607, #2230, #2231.
Mapping: Fixes Merging of default analyzer

Fixed behaviour where two representations of the default index analyzer weren't being treated as equivalent. Added REST test to confirm fix.

Closes #2716