implement Ticket#merge

closes #154
v1.3.2
Use file content_type when file is ActionDispatch::Http::UploadedFile
Keep ActionDispatch logic in one place
Merge pull request #155 from zendesk/vkmita/HC-2715-fix-content-type-upload

Use file content_type when file is ActionDispatch::Http::UploadedFile
v1.3.3
App::Installation is actually AppInstallation; close #157
[VOICE-1189] Add Voice resources to api client
Merge pull request #158 from zendesk/scott2211/voice-1189-add-voice-resources

[VOICE-1189] Add Voice resources to api client
v1.3.4