Space before the type declartion in route throws a 404

[parz3val]

Adding a space before parameter type in the route will throw a 404, but will not give any runtime errors.

Reproduction:

async def homepage(request):
    return JSONResponse({'hello': 'world'})

async def homepage(request):
    return JSONResponse({'hello': 'world'})


app = Starlette(debug=True, routes=[
    Route('/mypath/{name:str}', homepage),
])


app = Starlette(debug=True, routes=[
    Route('/mypath/{name:str}', homepage),
    Route('/mypath2/{name: str}', homepage), # this route will not be registered and throw 404.
])

I don't know if its intentional, but I somehow ended up thinking I could provide types in a same way as I do in python, and ended up shipping this bug in production.

I was thinking it would be nice to allow the space before the type declaration or throw an error when serving the app if user adds space in the route.

Here's what the current route declaration allows

# Match parameters in URL paths, eg. '{param}', and '{param:int}'
PARAM_REGEX = re.compile("{([a-zA-Z_][a-zA-Z0-9_]*)(:[a-zA-Z_][a-zA-Z0-9_]*)?}")

Here's what I was thinking we could do

# Match parameters in URL paths, eg. '{param}',  '{param:int}', and '{param: int}'
PARAM_REGEX = re.compile(r"{([a-zA-Z_][a-zA-Z0-9_]*)(?:\: ?([a-zA-Z_][a-zA-Z0-9_]*))?}")

If its up for discussion, I would like to open a PR for this as we've already added this feature for our frozen fork of Starlette.

[parz3val]

We could also throw a syntax error like Django, which I am also open to. But django does things like this

urlpatterns = [
    path("articles/2003/", views.special_case_2003),
    path("articles/<int:year>/", views.year_archive),
    path("articles/<int:year>/<int:month>/", views.month_archive),

And throws error if users try path("articles/<int: year>", views.smth), It'll throw an error saying there's a whitespace in the path.
The exact error is something like this

in _route_to_regex
    raise ImproperlyConfigured(
django.core.exceptions.ImproperlyConfigured: URL route 'articles/<int: year>' cannot contain whitespace in angle brackets <…>.