Maintain case-insensitivity when merging multiple matchers (#213)

When given two regexps, Regexp.union preserves the options set (or not
set) on each regex; this meant that none of the multiline (m),
case-insensitivity (i), or extended syntax (x) options were set.  Our
regexps are written expecting the m, i, and x options were set on all of
them, so we need to make sure that we preserve that behavior.

spec/models/glitch/keyword_mute_spec.rb

@@ -60,6 +60,13 @@ RSpec.describe Glitch::KeywordMute, type: :model do
         expect(matcher =~ 'This is a HOT take').to be_truthy
       end
 
+      it 'maintains case-insensitivity when combining keywords into a single matcher' do
+        Glitch::KeywordMute.create!(account: alice, keyword: 'hot')
+        Glitch::KeywordMute.create!(account: alice, keyword: 'cold')
+
+        expect(matcher =~ 'This is a HOT take').to be_truthy
+      end
+
       it 'matches keywords surrounded by non-alphanumeric ornamentation' do
         Glitch::KeywordMute.create!(account: alice, keyword: 'hot')