Deal with regex match groups in excerpt

[garethrees]

Original implementation has bugs if the regex contains a match group.

Example:

excerpt('This is a beautiful? morning', /\b(beau\w*)\b/i, :radius => 5)
Expected: "...is a beautiful? mor..."
Actual: "...is a beautifulbeaut..."

The original phrase was being converted to a regex and returning the text
either side of the phrase as expected:

'This is a beautiful? morning'.split(/beautiful/i, 2)
# => ["This is a ", "? morning"]

When we have a match with groups the match is returned in the array.

Quoting the ruby docs: "If pattern is a Regexp, str is divided where the
pattern matches. [...] If pattern contains groups, the respective matches will
be returned in the array as well."

'This is a beautiful? morning'.split(/\b(beau\w*)\b/iu, 2)
# => ["This is a ", "beautiful", "? morning"]

If we assume we want to split on the first match – this fix makes that
assumption – we can pass the already assigned phrase variable as the place
to split (because we already know that a match exists from line 168).

Originally spotted by Louise Crow (@crowbot) at
mysociety/alaveteli#1557