fix: Respond to /api requests as JSON by default

If header 'Accept: application/json' isn't set, the failure
responses to /api endpoints is HTML. Success responses are of type JSON.

(cherry picked from commit fcf63622bc)

frappe/app.py

@@ -201,12 +201,20 @@ def handle_exception(e):
 	response = None
 	http_status_code = getattr(e, "http_status_code", 500)
 	return_as_message = False
+	accept_header = frappe.get_request_header("Accept") or ""
+	respond_as_json = (
+		frappe.get_request_header('Accept')
+		and (frappe.local.is_ajax or 'application/json' in accept_header)
+		or (
+			frappe.local.request.path.startswith("/api/") and not accept_header.startswith("text")
+		)
+	)
 
 	if frappe.conf.get('developer_mode'):
 		# don't fail silently
 		print(frappe.get_traceback())
 
-	if frappe.get_request_header('Accept') and (frappe.local.is_ajax or 'application/json' in frappe.get_request_header('Accept')):
+	if respond_as_json:
 		# handle ajax responses first
 		# if the request is ajax, send back the trace or error message
 		response = frappe.utils.response.report_error(http_status_code)