Highlighting can return excerpt with no highlights

You can configure the highlighting api to return an excerpt of a field
even if there wasn't a match on the field.

The FVH makes excerpts from the beginning of the string to the first
boundary character after the requested length or the boundary_max_scan,
whichever comes first.  The Plain highlighter makes excerpts from the
beginning of the string to the end of the last token before the requested
length.

Closes #1171
Minor changes to no_match_size highlight parameter and highlight tests (#1171)

- Randomly store the field to highlight
- Updated test to use common assertions
- Restored previously commented out testCommonTermsQuery
[DOCS] rephrased docs for highlight no_match_size parameter