Fix search 'autocomplete' behaviour

Search results will no longer disappear and reappear again as a user is
typing a word in.

Removing the stemmer from the search index pipeline stops the search
term from being truncated when a search is made.

This fixes alphagov#222.

Description of issue
--------------------

I noticed that certain searches have surprising behaviour as the search
term was typed in; for instance, when searching for "documentation":

- searching "d" gives 5 results
- searching "do" gives 0 results
- searching "doc", "docu", "docum", "docume", "documen", "document"
  gives 1 result
- searching "documenta", "documentat", "documentati", "documentatio"
  gives 0 results
- searching "documentation" gives 1 result

This is surprising because search is incremental, meaning the results
change a lot while typing a word.

It is could also be confusing when searching for a particular long word;
for instance, if someone was searching for references to "documentation"
and there are a lot of results for "document" but none for "documenta"
they might think that means they are no uses of the word "documentation"
in the website.

Fix
---

This problem had already been identified in the Design System website,
see commit alphagov/govuk-design-system@531d3c3. Thanks @36degrees for
flagging that this fix existed.

The fix is to remove the stemmer from the lunr pipeline. This is easily
done with the fork of [middleman-search] that the tech-docs-gem uses.

This commit also adds a simple test to prevent future changes breaking
this functionality.

[middleman-search]: https://github.com/alphagov/middleman-search

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- [#236: Fix search 'autocomplete' behaviour](https://github.com/alphagov/tech-docs-gem/pull/236)
+
 ## 2.4.2
 
 - [#251 Fix missing `<ul>` in single page navigation](https://github.com/alphagov/tech-docs-gem/pull/251)

lib/govuk_tech_docs.rb

@@ -113,7 +113,7 @@ def active_page(page_path)
           url:     { index: false, store: true },
         }
 
-        search.pipeline_remove = %w[stopWordFilter]
+        search.pipeline_remove = %w[stemmer stopWordFilter]
 
         search.tokenizer_separator = '/[\s\-/]+/'
       end

spec/javascripts/search-spec.js

@@ -75,6 +75,14 @@ describe('Search', function () {
       ]
       expect(searchResults).toEqual(expectedResults)
     })
+
+    it('autocompletes words', function () {
+      module.search('documenta', function (incomplete) {
+        module.search('documentation', function (complete) {
+          expect(incomplete).toEqual(complete)
+        })
+      })
+    })
   })
 
   describe('the processContent method', function () {