Fix methods on the same URL getting ignored

[dvcrn]

When multiple methods were usable on the same URL, only the first in order would get parsed and the rest ignored.

Missing methods now show up correctly:

iex(8)> ExOpenAI.Assistants.
create_assistant/1         create_assistant/2
create_assistant_file/2    create_assistant_file/3
delete_assistant/1         delete_assistant/2
delete_assistant_file/2    delete_assistant_file/3
get_assistant/1            get_assistant/2
get_assistant_file/2       get_assistant_file/3
list_assistant_files/1     list_assistant_files/2
list_assistants/0          list_assistants/1
modify_assistant/1         modify_assistant/2

Fixes #15