Improve integration test helper functions (#2049)

Set request headers in helper functions, and new helper for requests with string-formatted URLs

integrations/repo_migrate_test.go

@@ -30,7 +30,6 @@ func testRepoMigrate(t testing.TB, session *TestSession, cloneAddr, repoName str
 		"repo_name":  repoName,
 	},
 	)
-	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
 	resp = session.MakeRequest(t, req)
 	assert.EqualValues(t, http.StatusFound, resp.HeaderCode)