Set a default of 0 for time_estimate column on Issuables.

This is to have consistency with the #time_spent method on Issuables
that returns 0 when the user has not entered any info about it.

db/migrate/20161030005533_add_estimate_to_issuables.rb

@@ -24,7 +24,7 @@ class AddEstimateToIssuables < ActiveRecord::Migration
   # disable_ddl_transaction!
 
   def change
-    add_column :issues, :time_estimate, :integer
-    add_column :merge_requests, :time_estimate, :integer
+    add_column :issues, :time_estimate, :integer, default: 0
+    add_column :merge_requests, :time_estimate, :integer, default: 0
   end
 end

db/schema.rb

@@ -560,7 +560,7 @@ ActiveRecord::Schema.define(version: 20161106185620) do
     t.integer "lock_version"
     t.text "title_html"
     t.text "description_html"
-    t.integer "time_estimate"
+    t.integer "time_estimate", default: 0
   end
 
   add_index "issues", ["assignee_id"], name: "index_issues_on_assignee_id", using: :btree
@@ -757,7 +757,7 @@ ActiveRecord::Schema.define(version: 20161106185620) do
     t.integer "lock_version"
     t.text "title_html"
     t.text "description_html"
-    t.integer "time_estimate"
+    t.integer "time_estimate", default: 0
   end
 
   add_index "merge_requests", ["assignee_id"], name: "index_merge_requests_on_assignee_id", using: :btree