Avoid a TypeError when initializing MergeRequest JS class with no arg

Without this sane default you would get the following error when you
tried to instantiate a new MergeRequest object with no argument (i.e.
`new MergeRequest();`):

TypeError: undefined is not an object (evaluating 'this.opts.action')

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/assets/javascripts/merge_request.js.coffee

@@ -9,7 +9,7 @@ class @MergeRequest
   # Options:
   #   action - String, current controller action
   #
-  constructor: (@opts) ->
+  constructor: (@opts = {}) ->
     this.$el = $('.merge-request')
 
     this.$('.show-all-commits').on 'click', =>

spec/javascripts/merge_request_spec.js.coffee

@@ -6,7 +6,7 @@ describe 'MergeRequest', ->
 
     beforeEach ->
       fixture.load('merge_requests_show.html')
-      @merge = new MergeRequest({})
+      @merge = new MergeRequest()
 
     it 'modifies the Markdown field', ->
       spyOn(jQuery, 'ajax').and.stub()